package Contextual::Return;

# Fake out CORE::GLOBAL::caller and Carp very early...
BEGIN {
    no warnings 'redefine';

    *CORE::GLOBAL::caller = sub {
        my ($uplevels) = shift || 0;
        return CORE::caller($uplevels + 2 + $Contextual::Return::uplevel)
            if $Contextual::Return::uplevel;
        return CORE::caller($uplevels + 1);
    };

    use Carp;
    my $real_carp  = *Carp::carp{CODE};
    my $real_croak = *Carp::croak{CODE};

    *Carp::carp = sub {
        goto &{$real_carp} if !$Contextual::Return::uplevel;
        warn _in_context(@_);
    };

    *Carp::croak = sub {
        goto &{$real_croak} if !$Contextual::Return::uplevel;
        die _in_context(@_);
    };

}

our $VERSION = '0.003001';

use warnings;
use strict;

sub _in_context {
    my $msg = join q{}, @_;

    # Start looking in caller...
    my $stack_frame = 1;
    my ($package, $file, $line, $sub) = CORE::caller($stack_frame++);

    my ($orig_package, $prev_package) = ($package) x 2;
    my $LOC = qq{at $file line $line}; 

    # Walk up stack...
    STACK_FRAME:
    while (1) {
        my ($package, $file, $line, $sub) = CORE::caller($stack_frame++);

        # Fall off the top of the stack...
        last if !defined $package;

        # Ignore this module (and any helpers)...
        next STACK_FRAME if $package =~ m{^Contextual::Return}xms;

        # Track the call up the stack...
        $LOC = qq{at $file line $line}; 

        # Ignore transitions within original caller...
        next STACK_FRAME
            if $package eq $orig_package && $prev_package eq $orig_package;

        # If we get a transition out of the original package, we're there...
        last STACK_FRAME;
    }

    # Insert location details...
    $msg =~ s/<LOC>/$LOC/g or $msg =~ s/[^\S\n]*$/ $LOC/;
    $msg =~ s/$/\n/;
    return $msg;
}

# Indentation corresponds to inherited fall-back relationships...
my @CONTEXTS = qw(
    DEFAULT
        VOID
        NONVOID
            LIST
            SCALAR
                VALUE
                    STR
                    NUM
                    BOOL
                        PUREBOOL
                REF
                    SCALARREF
                    ARRAYREF
                    CODEREF
                    HASHREF
                    GLOBREF
                    OBJREF
                        METHOD
);

my %attrs_of;

my @ALL_EXPORTS = (
    @CONTEXTS,
    qw(
        LAZY       RESULT      RVALUE      METHOD     FAIL
        FIXED      RECOVER     LVALUE      RETOBJ     FAIL_WITH
        ACTIVE     CLEANUP     NVALUE 
    )
);

my %STD_NAME_FOR = map { $_ => $_ } @ALL_EXPORTS;

sub import {
    # Load utility module for failure handlers...
    if (require Contextual::Return::Failure) {
        *FAIL      = \&Contextual::Return::Failure::_FAIL;
        *FAIL_WITH = \&Contextual::Return::Failure::_FAIL_WITH;
    }

    # Don't need the package name...
    shift @_;

    # If args, export nothing by default; otherwise export all...
    my %exports = @_ ? () : %STD_NAME_FOR;

    # All args are export either selectors and/or renamers...
    while (my $selector = shift @_) {
        my $next_arg = $_[0];
        my $renamer = (defined $next_arg
                    && !ref $next_arg
                    && !exists $STD_NAME_FOR{$next_arg})
                        ? shift(@_)
                        : undef;
        %exports = (%exports, _add_exports_for($selector, $renamer));
    }

    # Loop through possible exports, exporting anything requested...
    my $caller = CORE::caller;
    EXPORT:
    for my $subname (keys %exports) {
        no strict qw( refs );
        *{$caller.'::'.$exports{$subname}} = \&{$subname};
    }
};

sub _add_exports_for {
    my ($selector, $renamer) = @_;

    # If no renamer, use original name...
    $renamer ||= '%s';

    # Handle different types of selector...
    my $selector_type = ref($selector) || 'literal';

    # Array selector recursively export each element...
    if ($selector_type eq 'ARRAY') {
        return map { _add_exports_for($_,$renamer) } @{$selector};
    }
    elsif ($selector_type eq 'Regexp') {
        my @selected = grep {/$selector/} @ALL_EXPORTS;
        if (!@selected) {
            Carp:carp("use Contextual::Return $selector didn't export anything");
        }
        return map { $_ => sprintf($renamer, $_) } @selected;
    }
    elsif ($selector_type eq 'literal') {
        Carp::croak "Can't export $selector: no such handler"
            if !exists $STD_NAME_FOR{$selector};
        return ( $selector => sprintf($renamer, $selector) );
    }
    else {
        Carp::croak "Can't use $selector_type as export specifier";
    }
}


# Let handlers access the result object they're inside...

sub RETOBJ() {
    our $__RETOBJ__;
    return $__RETOBJ__;
}


use Scalar::Util qw( refaddr );

# Override return value in a C::R handler...
sub RESULT(;&) {
    my ($block) = @_;

    # Determine call context and arg list...
    my $context;
    my $args = do { package DB; $context=(CORE::caller 1)[5]; \@DB::args };

    # No args -> return appropriate value...
    if (!@_) {
        return $context ? @{ $Contextual::Return::__RESULT__ || [] }
                        :    $Contextual::Return::__RESULT__->[0]
                        ;
    }

    # Hide from caller() and the enclosing eval{}...
    
    # Evaluate block in context and cache result...
    local $Contextual::Return::uplevel = $Contextual::Return::uplevel+1;
    $Contextual::Return::__RESULT__
        =         $context  ? [        $block->(@{$args})      ]
        : defined $context  ? [ scalar $block->(@{$args}) ]
        :                     do {     $block->(@{$args}); [] }
        ;

    return;
}

sub RVALUE(&;@) :lvalue;
sub LVALUE(&;@) :lvalue;
sub NVALUE(&;@) :lvalue;

my %opposite_of = (
    'RVALUE' => 'LVALUE or NVALUE',
    'LVALUE' => 'RVALUE or NVALUE',
    'NVALUE' => 'LVALUE or RVALUE',
);


BEGIN {
    for my $subname (qw( RVALUE LVALUE NVALUE) ) {
        no strict 'refs';
        *{$subname} = sub(&;@) :lvalue {    # (handler, return_lvalue);
            my $handler = shift;
            my $impl;
            my $args = do{ package DB; ()=CORE::caller(1); \@DB::args };
            if (@_==0) {
                $impl = tie $_[0], 'Contextual::Return::Lvalue',
                    $subname => $handler, args=>$args;
            }
            elsif (@_==1 and $impl = tied $_[0]) {
                die _in_context "Can't install two $subname handlers"
                    if exists $impl->{$subname};
                $impl->{$subname} = $handler;
            }
            else {
                my $vals = join q{, }, map { tied $_    ? keys %{tied $_}
                                           : defined $_ ? $_
                                           :              'undef'
                                           } @_;
                die _in_context "Expected a $opposite_of{$subname} block ",
                                "after the $subname block <LOC> ",
                                "but found instead: $vals\n";
            }

            # Handle void context calls...
            if (!defined wantarray && $impl->{NVALUE}) {
                # Fake out caller() and Carp...
                local $Contextual::Return::uplevel = 1;

                # Call and clear handler...
                local $Contextual::Return::__RETOBJ__ = $impl;
                $impl->{NVALUE}( @{$impl->{args}} );
                delete $impl->{NVALUE};
            }
            $_[0];
        }
    }
}

sub FIXED ($) {
    my ($crv) = @_;
    $attrs_of{refaddr $crv}{FIXED} = 1;
    return $crv;
}

sub ACTIVE ($) {
    my ($crv) = @_;
    $attrs_of{refaddr $crv}{ACTIVE} = 1;
    return $crv;
}

sub LIST (;&$) {
    my ($block, $crv) = @_;

    # Handle simple context tests...
    return !!(CORE::caller 1)[5] if !@_;

    # Ensure we have an object...
    my $attrs;
    if (!refaddr $crv) {
        my $args = do{ package DB; ()=CORE::caller(1); \@DB::args };
        my $subname = (CORE::caller(1))[3];
        $crv = bless \my $scalar, 'Contextual::Return::Value';
        $attrs = $attrs_of{refaddr $crv} = { args => $args, sub => $subname };
    }
    else {
        $attrs = $attrs_of{refaddr $crv};
    }
    local $Contextual::Return::__RETOBJ__ = $crv;

    # Handle repetitions...
    die _in_context "Can't install two LIST handlers"
        if exists $attrs->{LIST};

    # Identify contexts...
    my $wantarray = wantarray;
    use Want;
    $attrs->{want_pure_bool} ||= Want::want('BOOL');

    # Prepare for exception handling...
    my $recover = $attrs->{RECOVER};
    local $Contextual::Return::uplevel = 2;

    # Handle list context directly...
    if (wantarray) {
        local $Contextual::Return::__RESULT__;

        my @rv = eval { $block->(@{$attrs->{args}}) };
        if ($recover) {
            if (!$Contextual::Return::__RESULT__) {
                $Contextual::Return::__RESULT__ = [@rv];
            }
            () = $recover->(@{$attrs->{args}});
        }
        elsif ($@) {
            die $@;
        }

        return @rv if !$Contextual::Return::__RESULT__;
        return @{$Contextual::Return::__RESULT__};
    }

    # Handle void context directly...
    if (!defined $wantarray) {
        handler:
        for my $context (qw< VOID DEFAULT >) {
            my $handler = $attrs->{$context} or next;

            eval { $attrs->{$context}->(@{$attrs->{args}}) };
            if ($recover) {
                $recover->(@{$attrs->{args}});
            }
            elsif ($@) {
                die $@;
            }
            last;
        }
        return;
    }

    # Otherwise, cache handler...
    $attrs->{LIST} = $block;
    return $crv;
}


sub VOID (;&$) {
    my ($block, $crv) = @_;

    # Handle simple context tests...
    return !defined( (CORE::caller 1)[5] ) if !@_;

    # Ensure we have an object...
    my $attrs;
    if (!refaddr $crv) {
        my $args = do{ package DB; ()=CORE::caller(1); \@DB::args };
        my $subname = (CORE::caller(1))[3];
        $crv = bless \my $scalar, 'Contextual::Return::Value';
        $attrs = $attrs_of{refaddr $crv} = { args => $args, sub => $subname };
    }
    else {
        $attrs = $attrs_of{refaddr $crv};
    }
    local $Contextual::Return::__RETOBJ__ = $crv;

    # Handle repetitions...
    die _in_context "Can't install two VOID handlers"
        if exists $attrs->{VOID};

    # Identify contexts...
    my $wantarray = wantarray;
    use Want;
    $attrs->{want_pure_bool} ||= Want::want('BOOL');

    # Prepare for exception handling...
    my $recover = $attrs->{RECOVER};
    local $Contextual::Return::uplevel = 2;

    # Handle list context directly, if possible...
    if (wantarray) {
        local $Contextual::Return::__RESULT__;
        # List or ancestral handlers...
        handler:
        for my $context (qw(LIST VALUE NONVOID DEFAULT)) {
            my $handler = $attrs->{$context} or next;

            my @rv = eval { $handler->(@{$attrs->{args}}) };
            if ($recover) {
                if (!$Contextual::Return::__RESULT__) {
                    $Contextual::Return::__RESULT__ = [@rv];
                }
                () = $recover->(@{$attrs->{args}});
            }
            elsif ($@) {
                die $@;
            }

            return @rv if !$Contextual::Return::__RESULT__;
            return @{$Contextual::Return::__RESULT__};
        }
        # Convert to list from arrayref handler...
        if (my $handler = $attrs->{ARRAYREF}) {
            my $array_ref = eval { $handler->(@{$attrs->{args}}) };

            if ($recover) {
                if (!$Contextual::Return::__RESULT__) {
                    $Contextual::Return::__RESULT__ = [$array_ref];
                }
                scalar $recover->(@{$attrs->{args}});
            }
            elsif ($@) {
                die $@;
            }

            # Array ref may be returned directly, or via RESULT{}...
            $array_ref = $Contextual::Return::__RESULT__->[0]
                if $Contextual::Return::__RESULT__;

            return @{$array_ref} if (ref $array_ref||q{}) eq 'ARRAY';
        }
        # Return scalar object as one-elem list, if possible...
        handler:
        for my $context (qw(BOOL STR NUM VALUE SCALAR LAZY)) {
            return $crv if exists $attrs->{$context};
        }
        $@ = _in_context "Can't call $attrs->{sub} in list context";
        if ($recover) {
            () = $recover->(@{$attrs->{args}});
        }
        else {
            die $@;
        }
    }

    # Handle void context directly...
    if (!defined $wantarray) {
        eval { $block->(@{$attrs->{args}}) };

        if ($recover) {
            $recover->(@{$attrs->{args}});
        }
        elsif ($@) {
            die $@;
        }

        return;
    }

    # Otherwise, cache handler...
    $attrs->{VOID} = $block;
    return $crv;
}

for my $context (qw( SCALAR NONVOID )) {
    no strict qw( refs );
    *{$context} = sub (;&$) {
        my ($block, $crv) = @_;

        # Handle simple context tests...
        if (!@_) {
            my $callers_context = (CORE::caller 1)[5];
            return defined $callers_context
                && ($context eq 'NONVOID' || !$callers_context);
        }

        # Ensure we have an object...
        my $attrs;
        if (!refaddr $crv) {
            my $args = do{ package DB; ()=CORE::caller(1); \@DB::args };
            my $subname = (CORE::caller(1))[3];
            $crv = bless \my $scalar, 'Contextual::Return::Value';
            $attrs = $attrs_of{refaddr $crv}
                    = { args => $args, sub => $subname };
        }
        else {
            $attrs = $attrs_of{refaddr $crv};
        }
        local $Contextual::Return::__RETOBJ__ = $crv;

        # Make sure this block is a possibility too...
        die _in_context "Can't install two $context handlers"
            if exists $attrs->{$context};
        $attrs->{$context} = $block;

        # Identify contexts...
        my $wantarray = wantarray;
        use Want;
        $attrs->{want_pure_bool} ||= Want::want('BOOL');

        # Prepare for exception handling...
        my $recover = $attrs->{RECOVER};
        local $Contextual::Return::uplevel = 2;

        # Handle list context directly, if possible...
        if (wantarray) {
            local $Contextual::Return::__RESULT__;

            # List or ancestral handlers...
            handler:
            for my $context (qw(LIST VALUE NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                my @rv = eval { $handler->(@{$attrs->{args}}) };
                if ($recover) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [@rv];
                    }
                    () = $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                return @rv if !$Contextual::Return::__RESULT__;
                return @{$Contextual::Return::__RESULT__};
            }
            # Convert to list from arrayref handler...
            if (my $handler = $attrs->{ARRAYREF}) {

                my $array_ref = eval { $handler->(@{$attrs->{args}}) };
                if ($recover) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$array_ref];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                # Array ref may be returned directly, or via RESULT{}...
                $array_ref = $Contextual::Return::__RESULT__->[0]
                    if $Contextual::Return::__RESULT__;

                return @{$array_ref} if (ref $array_ref||q{}) eq 'ARRAY';
            }
            # Return scalar object as one-elem list, if possible...
            handler:
            for my $context (qw(BOOL STR NUM VALUE SCALAR LAZY)) {
                return $crv if exists $attrs->{$context};
            }
            die _in_context "Can't call $attrs->{sub} in list context";
        }

        # Handle void context directly...
        if (!defined $wantarray) {
            handler:
            for my $context (qw< VOID DEFAULT >) {
                my $handler = $attrs->{$context} or next;

                eval { $handler->(@{$attrs->{args}}) };
                if ($recover) {
                    $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                last;
            }
            return;
        }

        # Otherwise, defer evaluation by returning an object...
        return $crv;
    }
}

for my $context_name (@CONTEXTS, qw< RECOVER _internal_LIST CLEANUP >) {
    next if $context_name eq 'LIST'       # These
         || $context_name eq 'VOID'       #  four
         || $context_name eq 'SCALAR'     #   handled
         || $context_name eq 'NONVOID';   #    separately

    no strict qw( refs );
    *{$context_name} = sub (&;$) {
        my ($block, $crv) = @_;

        # Ensure we have an object...
        my $attrs;
        if (!refaddr $crv) {
            my $args = do{ package DB; ()=CORE::caller(1); \@DB::args };
            my $subname = (CORE::caller(1))[3];
            $crv = bless \my $scalar, 'Contextual::Return::Value';
            $attrs = $attrs_of{refaddr $crv}
                     = { args => $args, sub => $subname };
        }
        else {
            $attrs = $attrs_of{refaddr $crv};
        }
        local $Contextual::Return::__RETOBJ__ = $crv;

        # Make sure this block is a possibility too...
        if ($context_name ne '_internal_LIST') {
            die _in_context "Can't install two $context_name handlers"
                if exists $attrs->{$context_name};
            $attrs->{$context_name} = $block;
        }

        # Identify contexts...
        my $wantarray = wantarray;
        use Want;
        $attrs->{want_pure_bool} ||= Want::want('BOOL');

        # Prepare for exception handling...
        my $recover = $attrs->{RECOVER};
        local $Contextual::Return::uplevel = 2;

        # Handle list context directly, if possible...
        if (wantarray) {
            local $Contextual::Return::__RESULT__
                = $context_name eq 'RECOVER' ? $Contextual::Return::__RESULT__
                :                              undef
                ;

            # List or ancestral handlers...
            handler:
            for my $context (qw(LIST VALUE NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                my @rv = eval { $handler->(@{$attrs->{args}}) };
                if ($recover) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [@rv];
                    }
                    () = $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                return @rv if !$Contextual::Return::__RESULT__;
                return @{$Contextual::Return::__RESULT__};
            }
            # Convert to list from arrayref handler...
            if (my $handler = $attrs->{ARRAYREF}) {
                local $Contextual::Return::uplevel = 2;

                # Array ref may be returned directly, or via RESULT{}...
                my $array_ref = eval { $handler->(@{$attrs->{args}}) };
                if ($recover) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$array_ref];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                $array_ref = $Contextual::Return::__RESULT__->[0]
                    if $Contextual::Return::__RESULT__;

                return @{$array_ref} if (ref $array_ref||q{}) eq 'ARRAY';
            }
            # Return scalar object as one-elem list, if possible...
            handler:
            for my $context (qw(BOOL STR NUM VALUE SCALAR LAZY)) {
                return $crv if exists $attrs->{$context};
            }
            $@ = _in_context "Can't call $attrs->{sub} in list context";
            if ($recover) {
                () = $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        }

        # Handle void context directly...
        if (!defined $wantarray) {
            handler:
            for my $context (qw(VOID DEFAULT)) {
                next if !$attrs->{$context};

                eval { $attrs->{$context}->(@{$attrs->{args}}) };

                if ($recover) {
                    $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                last;
            }
            return;
        }

        # Otherwise, defer evaluation by returning an object...
        return $crv;
    }
}

# Alias LAZY to SCALAR...
*LAZY = *SCALAR;


# Set $Data::Dumper::Freezer to 'Contextual::Return::FREEZE' to be able to
# dump contextual return values...

my %operator_impl;

my $no_handler_message = qr{
    ^ Can't [ ] call [ ] .*? [ ] in [ ] [\w]+ [ ] context
  | ^ [\w:]+ [ ] can't [ ] return [ ] a [ ] \w+ [ ] reference
}xms;

sub _flag_self_ref_in {
    my ($data_ref, $obj_ref) = @_;
    my $type = ref $data_ref;
    return if !$type;
    for my $value ( $type eq 'SCALAR' ? ${$data_ref} : @{$data_ref} ) {
        no warnings 'numeric';
        if ($value == $obj_ref) {
            $value = '<<<self-reference>>>';
        }
    }
}

sub FREEZE {
    my ($self) = @_;
    my $attrs_ref = $attrs_of{refaddr $self};
    my $args_ref  = $attrs_ref->{args};

    my @no_handler;

    # Call appropriate operator handler, defusing and recording exceptions...
    my $overloaded = sub {
        my ($context, $op) = @_;

        # Try the operator...
        my $retval = eval { $operator_impl{$op}->($self,@{$args_ref}) };

        # Detect and report internal exceptions...
        if (my $exception = $@) {
            if ($exception =~ $no_handler_message) {
                push @no_handler, $context;
                return ();
            }
            return { $context => "<<<Throws exception: $exception>>>" };
        }

        # Detect self-referential overloadings (to avoid infinite recursion)...
        {
            no warnings 'numeric';
            if (ref $retval eq 'REF' && ${$retval} == ${$self}) {
                return { $context => "<<<self-reference>>>" };
            }
        }

        # Normal return of contextual value labelled by context...
        return { $context => $retval };
    };

    my @values;

    # Where did this value originate?
    push @values, { ISA  => 'Contextual::Return::Value' };
    push @values, { FROM => $attrs_ref->{sub} };

    # Does it return a value in void context?
    if (exists $attrs_ref->{VOID} || exists $attrs_ref->{DEFAULT}) {
        push @values, { VOID => undef };
    }
    else {
        push @no_handler, 'VOID';
    }

    # Generate list context value by "pretend" LIST handler...
    push @values, { LIST => [ _internal_LIST(sub{}, $self) ] };
        _flag_self_ref_in($values[-1]{LIST}, $self);

    # Generate scalar context values by calling appropriate handler...
    push @values, $overloaded->( STR       => q{""}  );
    push @values, $overloaded->( NUM       => '0+'   );
    push @values, $overloaded->( BOOL      => 'bool' );
    push @values, $overloaded->( SCALARREF => '${}'  );
        _flag_self_ref_in($values[-1]{SCALARREF}, $self);
    push @values, $overloaded->( ARRAYREF  => '@{}'  );
        _flag_self_ref_in($values[-1]{ARRAYREF}, $self);
    push @values, $overloaded->( CODEREF   => '&{}'  );
    push @values, $overloaded->( HASHREF   => '%{}'  );
    push @values, $overloaded->( GLOBREF   => '*{}'  );

    # Are there handlers for various "generic" super-contexts...
    my @fallbacks = grep { $attrs_ref->{$_} }
                       qw< DEFAULT NONVOID SCALAR VALUE REF RECOVER >;
    
    push @values, { NO_HANDLER => \@no_handler };
    push @values, { FALLBACKS  => \@fallbacks  };

    # Temporarily replace object being dumped, by values found...
    $_[0] = \@values;
}

# Call this method on a contextual return value object to debug it...

sub DUMP {
    if (require Data::Dumper) {
        my ($crv) = @_;
        FREEZE($crv);
        return Data::Dumper::Dumper($crv);
    }
    else {
        Carp::carp("Can't DUMP contextual return value (no Data::Dumper!)");
        return;
    }
}


package Contextual::Return::Value;
BEGIN { *_in_context = *Contextual::Return::_in_context; }
use Scalar::Util qw( refaddr );

BEGIN {
    %operator_impl = (
        q{""} => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            handler:
            for my $context (qw(STR SCALAR LAZY VALUE NONVOID DEFAULT NUM)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            $@ = _in_context "Can't call $attrs->{sub} in string context";
            if (my $recover = $attrs->{RECOVER}) {
                scalar $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        },

        q{0+} => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            handler:
            for my $context (qw(NUM SCALAR LAZY VALUE NONVOID DEFAULT STR)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            $@ = _in_context "Can't call $attrs->{sub} in numeric context";
            if (my $recover = $attrs->{RECOVER}) {
                scalar $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        },

        q{bool} => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};

            # Handle Calls in Pure Boolean context...
            my @PUREBOOL = $attrs->{want_pure_bool} ? ('PUREBOOL') : ();
            $attrs->{want_pure_bool} = 0;

            handler:
            for my $context (@PUREBOOL, qw(BOOL SCALAR LAZY VALUE NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $outer_sig_warn = $SIG{__WARN__};
                local $SIG{__WARN__}
                    = sub{ return if $_[0] =~ /^Exiting \S+ via next/;
                           goto &{$outer_sig_warn} if $outer_sig_warn;
                           warn @_;
                      };
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            $@ = _in_context "Can't call $attrs->{sub} in boolean context";
            if (my $recover = $attrs->{RECOVER}) {
                scalar $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        },
        '${}' => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            handler:
            for my $context (qw(SCALARREF REF NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                # Catch bad behaviour...
                die _in_context "$context block did not return ",
                                "a suitable reference to the scalar dereference"
                        if ref($rv) ne 'SCALAR' && ref($rv) ne 'OBJ';

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            if ( $attrs->{FIXED} ) {
                $_[0] = \$self;
            }
            return \$self;
        },
        '@{}' => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            local $Contextual::Return::__RESULT__;
            handler:
            for my $context (qw(ARRAYREF REF)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                # Catch bad behaviour...
                die _in_context "$context block did not return ",
                                "a suitable reference to the array dereference"
                        if ref($rv) ne 'ARRAY' && ref($rv) ne 'OBJ';

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            handler:
            for my $context (qw(LIST VALUE NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::uplevel = 2;
                my @rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [@rv];
                    }
                    () = $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    @rv = @{$Contextual::Return::__RESULT__->[0]};
                }

                if ( $attrs->{FIXED} ) {
                    $_[0] = \@rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { @rv };
                }
                return \@rv;
            }
            return [ $self ];
        },
        '%{}' => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            handler:
            for my $context (qw(HASHREF REF NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                # Catch bad behaviour...
                die _in_context "$context block did not return ",
                                "a suitable reference to the hash dereference"
                        if ref($rv) ne 'HASH' && ref($rv) ne 'OBJ';

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            $@ = _in_context "$attrs->{sub} can't return a hash reference";
            if (my $recover = $attrs->{RECOVER}) {
                scalar $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        },
        '&{}' => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            handler:
            for my $context (qw(CODEREF REF NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                # Catch bad behaviour...
                die _in_context "$context block did not return ",
                                "a suitable reference to the subroutine dereference"
                        if ref($rv) ne 'CODE' && ref($rv) ne 'OBJ';

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            $@ = _in_context "$attrs->{sub} can't return a subroutine reference";
            if (my $recover = $attrs->{RECOVER}) {
                scalar $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        },
        '*{}' => sub {
            my ($self) = @_;
            local $Contextual::Return::__RETOBJ__ = $self;
            my $attrs = $attrs_of{refaddr $self};
            handler:
            for my $context (qw(GLOBREF REF NONVOID DEFAULT)) {
                my $handler = $attrs->{$context} or next;

                local $Contextual::Return::__RESULT__;
                local $Contextual::Return::uplevel = 2;
                my $rv = eval { $handler->(@{$attrs->{args}}) };

                if (my $recover = $attrs->{RECOVER}) {
                    if (!$Contextual::Return::__RESULT__) {
                        $Contextual::Return::__RESULT__ = [$rv];
                    }
                    scalar $recover->(@{$attrs->{args}});
                }
                elsif ($@) {
                    die $@;
                }

                if ($Contextual::Return::__RESULT__) {
                    $rv = $Contextual::Return::__RESULT__->[0];
                }

                # Catch bad behaviour...
                die _in_context "$context block did not return ",
                                "a suitable reference to the typeglob dereference"
                        if ref($rv) ne 'GLOB' && ref($rv) ne 'OBJ';

                if ( $attrs->{FIXED} ) {
                    $_[0] = $rv;
                }
                elsif ( !$attrs->{ACTIVE} ) {
                    $attrs->{$context} = sub { $rv };
                }
                return $rv;
            }
            $@ = _in_context "$attrs->{sub} can't return a typeglob reference";
            if (my $recover = $attrs->{RECOVER}) {
                scalar $recover->(@{$attrs->{args}});
            }
            else {
                die $@;
            }
        },
    );
}

use overload %operator_impl, fallback => 1;

sub DESTROY {
    my ($id) = refaddr shift;
    my $attrs = $attrs_of{$id};
    if (my $handler = $attrs->{CLEANUP}) {
        $handler->(@{ $attrs->{args} });
    }
    delete $attrs_of{$id};
    return;
}

my $NO_SUCH_METHOD = qr/\ACan't (?:locate|call)(?: class| object)? method/ms;

# Forward metainformation requests to actual class...
sub can {
    my ($invocant) = @_;
    # Only forward requests on actual C::R::V objects...
    if (ref $invocant) {
        our $AUTOLOAD = 'can';
        goto &AUTOLOAD;
    }

    # Refer requests on classes to actual class hierarchy...
    return $invocant->SUPER::can(@_[1..$#_]);
}

sub isa {
    # Only forward requests on actual C::R::V objects...
    my ($invocant) = @_;
    if (ref $invocant) {
        our $AUTOLOAD = 'isa';
        goto &AUTOLOAD;
    }

    # Refer requests on classes to actual class hierarchy...
    return $invocant->SUPER::isa(@_[1..$#_]);
}


sub AUTOLOAD {
    my ($self) = @_;
    our $AUTOLOAD;

    my ($requested_method) = $AUTOLOAD =~ m{ .* :: (.*) }xms ? $1 : $AUTOLOAD;

    my $attrs = $attrs_of{refaddr $self} || {};
    local $Contextual::Return::__RETOBJ__ = $self;

    # First, see if there is a method call handler...
    if (my $context_handler = $attrs->{METHOD}) {
        local $Contextual::Return::__RESULT__;
        local $Contextual::Return::uplevel = 2;
        my @method_handlers = eval { $context_handler->(@{$attrs->{args}}) };

        if (my $recover = $attrs->{RECOVER}) {
            if (!$Contextual::Return::__RESULT__) {
                $Contextual::Return::__RESULT__ = [\@method_handlers];
            }
            scalar $recover->(@{$attrs->{args}});
        }
        elsif ($@) {
            die $@;
        }

        if ($Contextual::Return::__RESULT__) {
            @method_handlers = @{$Contextual::Return::__RESULT__};
        }

        # Locate the correct method handler (if any)...
        MATCHER:
        while (my ($matcher, $method_handler) = splice @method_handlers, 0, 2) {

            if (ref($matcher) eq 'ARRAY') {
                next MATCHER
                    if !grep { $requested_method =~ $_ } @{$matcher};
            }
            elsif ($requested_method !~ $matcher) {
                next MATCHER;
            }

            shift;
            if (wantarray) {
                my @result = eval {
                    local $_ = $requested_method;
                    $method_handler->(@_);
                };
                die _in_context $@ if $@;
                return @result;
            }
            else {
                my $result = eval {
                    local $_ = $requested_method;
                    $method_handler->(@_);
                };
                die _in_context $@ if $@;
                return $result;
            }
        }
    }

    # Next, try to create an object on which to call the method...
    handler:
    for my $context (qw(OBJREF STR SCALAR LAZY VALUE NONVOID DEFAULT)) {
        my $handler = $attrs->{$context} or next;

        local $Contextual::Return::__RESULT__;
        local $Contextual::Return::uplevel = 2;
        my $object = eval { $handler->(@{$attrs->{args}}) };

        if (my $recover = $attrs->{RECOVER}) {
            if (!$Contextual::Return::__RESULT__) {
                $Contextual::Return::__RESULT__ = [$object];
            }
            scalar $recover->(@{$attrs->{args}});
        }
        elsif ($@) {
            die $@;
        }

        if ($Contextual::Return::__RESULT__) {
            $object = $Contextual::Return::__RESULT__->[0];
        }

        if ( $attrs->{FIXED} ) {
            $_[0] = $object;
        }
        elsif ( !$attrs->{ACTIVE} ) {
            $attrs->{$context} = sub { $object };
        }
        shift;

        if (wantarray) {
            my @result = eval { $object->$requested_method(@_) };
            my $exception = $@;
            return @result if !$exception;
            die _in_context $exception if $exception !~ $NO_SUCH_METHOD;
        }
        else {
            my $result = eval { $object->$requested_method(@_) };
            my $exception = $@;
            return $result if !$exception;
            die _in_context $exception if $exception !~ $NO_SUCH_METHOD;
        }
        $@ = _in_context "Can't call method '$requested_method' on $context value returned by $attrs->{sub}";
        if (my $recover = $attrs->{RECOVER}) {
            scalar $recover->(@{$attrs->{args}});
        }
        else {
            die $@;
        }
    }

    # Otherwise, the method cannot be called, so react accordingly...
    $@ = _in_context "Can't call method '$requested_method' on value returned by $attrs->{sub}";
    if (my $recover = $attrs->{RECOVER}) {
        return scalar $recover->(@{$attrs->{args}});
    }
    else {
        die $@;
    }
}

package Contextual::Return::Lvalue;

sub TIESCALAR {
    my ($package, @handler) = @_;
    return bless {@handler}, $package;
}

# Handle calls that are lvalues...
sub STORE {
    local *CALLER::_ = \$_;
    local *_         = \$_[1];
    local $Contextual::Return::uplevel = 1;
    local $Contextual::Return::__RESULT__;

    my $rv = $_[0]{LVALUE}( @{$_[0]{args}} );

    return $rv if !$Contextual::Return::__RESULT__;
    return $Contextual::Return::__RESULT__->[0];
}

# Handle calls that are rvalues...
sub FETCH {
    local $Contextual::Return::uplevel = 1;
    local $Contextual::Return::__RESULT__;

    my $rv = $_[0]{RVALUE} ? $_[0]{RVALUE}( @{$_[0]{args}} ) : undef;

    return $rv if !$Contextual::Return::__RESULT__;
    return $Contextual::Return::__RESULT__->[0];
}

sub DESTROY {};

1; # Magic true value required at end of module

__END__

=head1 NAME

Contextual::Return - Create context-senstive return values


=head1 VERSION

This document describes Contextual::Return version 0.003001


=head1 SYNOPSIS

    use Contextual::Return;
    use Carp;

    sub foo {
        return
            SCALAR { 'thirty-twelve' }
            LIST   { 1,2,3 }

            BOOL { 1 }
            NUM  { 7*6 }
            STR  { 'forty-two' }

            HASHREF  { {name => 'foo', value => 99} }
            ARRAYREF { [3,2,1] }

            GLOBREF  { \*STDOUT }
            CODEREF  { croak "Don't use this result as code!"; }
        ;
    }

    # and later...

    if (my $foo = foo()) {
        for my $count (1..$foo) {
            print "$count: $foo is:\n"
                . "  array: @{$foo}\n"
                . "  hash:  $foo->{name} => $foo->{value}\n"
                ;
        }
        print {$foo} $foo->();
    }

=head1 DESCRIPTION

Usually, when you need to create a subroutine that returns different values in
different contexts (list, scalar, or void), you write something like:

    sub get_server_status {
        my ($server_ID) = @_;

        # Acquire server data somehow...
        my %server_data = _ascertain_server_status($server_ID);

        # Return different components of that data,
        # depending on call context...
        if (wantarray()) {
            return @server_data{ qw(name uptime load users) };
        }
        if (defined wantarray()) {
            return $server_data{load};
        }
        if (!defined wantarray()) {
            carp 'Useless use of get_server_status() in void context';
            return;
        }
        else {
            croak q{Bad context! No biscuit!};
        }
    }

That works okay, but the code could certainly be more readable. In
its simplest usage, this module makes that code more readable by
providing three subroutines--C<LIST()>, C<SCALAR()>, C<VOID()>--that
are true only when the current subroutine is called in the
corresponding context:

    use Contextual::Return;

    sub get_server_status {
        my ($server_ID) = @_;

        # Acquire server data somehow...
        my %server_data = _ascertain_server_status($server_ID);

        # Return different components of that data
        # depending on call context...
        if (LIST)   { return @server_data{ qw(name uptime load users) } }
        if (SCALAR) { return $server_data{load}                         }
        if (VOID)   { print "$server_data{load}\n"                      }
        else        { croak q{Bad context! No biscuit!}                 }
    }

=head2 Contextual returns

Those three subroutines can also be used in another way: as labels on a
series of I<contextual return blocks> (collectively known as a I<contextual
return sequence>). When a context sequence is returned, it automatically
selects the appropriate contextual return block for the calling context.
So the previous example could be written even more cleanly as:

    use Contextual::Return;

    sub get_server_status {
        my ($server_ID) = @_;

        # Acquire server data somehow...
        my %server_data = _ascertain_server_status($server_ID);

        # Return different components of that data
        # depending on call context...
        return (
            LIST    { return @server_data{ qw(name uptime load users) } }
            SCALAR  { return $server_data{load}                         }
            VOID    { print "$server_data{load}\n"                      }
            DEFAULT { croak q{Bad context! No biscuit!}                 }
        );
    }

The context sequence automatically selects the appropriate block for each call
context.


=head2 Lazy contextual return values

C<LIST> and C<VOID> blocks are always executed during the C<return>
statement. However, scalar return blocks (C<SCALAR>, C<STR>, C<NUM>,
C<BOOL>, etc.) blocks are not. Instead, returning any of scalar block
types causes the subroutine to return an object that lazily
evaluates that block only when the return value is used.

This means that returning a C<SCALAR> block is a convenient way to
implement a subroutine with a lazy return value. For example:

    sub digest {
        return SCALAR {
            my ($text) = @_;
            md5($text);
        }
    }

    my $digest = digest($text);

    print $digest;   # md5() called only when $digest used as string

To better document this usage, the C<SCALAR> block has a synonym: C<LAZY>.

    sub digest {
        return LAZY {
            my ($text) = @_;
            md5($text);
        }
    }


=head2 Active contextual return values

Once a return value has been lazily evaluated in a given context, 
the resulting value is cached, and thereafter reused in that same context.

However, you can specify that, rather than being cached, the value
should be re-evaluated I<every> time the value is used:

     sub make_counter {
        my $counter = 0;
        return ACTIVE
            SCALAR   { ++$counter }
            ARRAYREF { [1..$counter] }
    } 
            
    my $idx = make_counter();
        
    print "$idx\n";      # 1
    print "$idx\n";      # 2
    print "[@$idx]\n";   # [1 2]
    print "$idx\n";      # 3
    print "[@$idx]\n";   # [1 2 3]

 
=head2 Semi-lazy contextual return values

Sometimes, single or repeated lazy evaluation of a scalar return value
in different contexts isn't what you really want. Sometimes what you
really want is for the return value to be lazily evaluated once only (the
first time it's used in any context), and then for that first value to
be reused whenever the return value is subsequently reevaluated in any
other context.

To get that behaviour, you can use the C<FIXED> modifier, which causes
the return value to morph itself into the actual value the first time it
is used. For example:

    sub lazy {
        return
            SCALAR { 42 }
            ARRAYREF { [ 1, 2, 3 ] }
        ;
    }

    my $lazy = lazy();
    print $lazy + 1;            # 43
    print "@{$lazy}";           # 1 2 3


    sub semilazy {
        return FIXED
            SCALAR { 42 }
            ARRAYREF { [ 1, 2, 3 ] }
        ;
    }

    my $semi = semilazy();
    print $semi + 1;            # 43
    print "@{$semi}";           # die q{Can't use string ("42") as an ARRAY ref}



=head2 Finer distinctions of scalar context

Because the scalar values returned from a context sequence are lazily
evaluated, it becomes possible to be more specific about I<what kind> of
scalar value should be returned: a boolean, a number, or a string. To support
those distinctions, Contextual::Return provides four extra context blocks:
C<NUM>, C<STR>, C<BOOL>, and C<PUREBOOL>:

    sub get_server_status {
        my ($server_ID) = @_;

        # Acquire server data somehow...
        my %server_data = _ascertain_server_status($server_ID);

        # Return different components of that data
        # depending on call context...
        return (
               LIST { @server_data{ qw(name uptime load users) }          }
           PUREBOOL { $_ = $server_data{uptime}; $server_data{uptime} > 0 }
               BOOL { $server_data{uptime} > 0                            }
               NUM  { $server_data{load}                                  }
               STR  { "$server_data{name}: $server_data{uptime}"          }
               VOID { print "$server_data{load}\n"                        }
            DEFAULT { croak q{Bad context! No biscuit!}                   }
        );
    }

With these in place, the object returned from a scalar-context call to
C<get_server_status()> now behaves differently, depending on how
it's used. For example:

    if ( my $status = get_server_status() ) {  # BOOL: True if uptime > 0
        $load_distribution[$status]++;         # INT:  Evaluates to load value
        print "$status\n";                     # STR:  Prints "name: uptime"
    }

    if (get_server_status()) {                 # PUREBOOL: also sets $_;
        print;                                 # ...which is then used here
    }

=head3 Boolean vs Pure Boolean contexts

There is a special subset of boolean contexts where the return value is being
used and immediately thrown away. For example, in the loop:

    while (get_data()) {
        ...
    }

the value returned by C<get_data()> is tested for truth and then discarded.
This is known as "pure boolean context". In contrast, in the loop:

    while (my $data = get_data()) {
        ...
    }

the value returned by C<get_data()> is first assigned to C<$data>, then
tested for truth. Because of the assignment, the return value is I<not>
discarded after the boolean test. This is ordinary "boolean context".

In Perl, pure boolean context is often associated with a special side-effect,
that does not occur in regular boolean contexts. For example:

    while (<>) {...}         # $_ set as side-effect of pure boolean context

    while ($v = <>) {...}    # $_ NOT set in ordinary boolean context

Contextual::Return supports this with a special subcase of C<BOOL> named
<PUREBOOL>. In pure boolean contexts, Contextual::Return will call a
C<PUREBOOL> handler if one has been defined, or fall back to a C<BOOL>
or C<SCALAR> handler if no C<PUREBOOL> handler exists. In ordinary
boolean contexts only the C<BOOL> or C<SCALAR> handlers are tried, even
if a C<PUREBOOL> handler is also defined.

Typically C<PUREBOOL> handlers are set up to have some side-effect (most
commonly: setting C<$_> or <$@>), like so:

    sub get_data {
        my ($succeeded, @data) = _go_and_get_data();

        return
            PUREBOOL { $_ = $data[0]; $succeeded; }
                BOOL {                $succeeded; }
              SCALAR {                $data[0];   }
                LIST {                @data;      }
    }

However, there is no requirement that they have side-effects. For example,
they can also be used to implement "look-but-don't-retrieve-yet" checking:

    sub get_data {
        my $data;
        return
            PUREBOOL {        _check_for_but_dont_get_data();   }
                BOOL { defined( $data ||= _go_and_get_data() ); }
                 REF {          $data ||= _go_and_get_data();   }
    }


=head2 Self-reference within handlers

Any handler can refer to the contextual return object it is part of, by
calling the C<RETOBJ()> function. This is particularly useful for C<PUREBOOL>
and C<LIST> handlers. For example:

    return 
        PUREBOOL { $_ = RETOBJ; next handler; }
            BOOL { !$failed;                  }
         DEFAULT { $data;                     };


=head2 Referential contexts

The other major kind of scalar return value is a reference.
Contextual::Return provides contextual return blocks that allow you to
specify what to (lazily) return when the return value of a subroutine is
used as a reference to a scalar (C<SCALARREF {...}>), to an array
(C<ARRAYREF {...}>), to a hash (C<HASHREF {...}>), to a subroutine
(C<CODEREF {...}>), or to a typeglob (C<GLOBREF {...}>).

For example, the server status subroutine shown earlier could be extended to
allow it to return a hash reference, thereby supporting "named return values":

    sub get_server_status {
        my ($server_ID) = @_;

        # Acquire server data somehow...
        my %server_data = _ascertain_server_status($server_ID);

        # Return different components of that data
        # depending on call context...
        return (
               LIST { @server_data{ qw(name uptime load users) }  }
               BOOL { $server_data{uptime} > 0                    }
                NUM { $server_data{load}                          }
                STR { "$server_data{name}: $server_data{uptime}"  }
               VOID { print "$server_data{load}\n"                }
            HASHREF { return \%server_data                        }
            DEFAULT { croak q{Bad context! No biscuit!}           }
        );
    }

    # and later...

    my $users = get_server_status->{users};


    # or, lazily...

    my $server = get_server_status();

    print "$server->{name} load = $server->{load}\n";


=head2 Interpolative referential contexts

The C<SCALARREF {...}> and C<ARRAYREF {...}> context blocks are
especially useful when you need to interpolate a subroutine into
strings. For example, if you have a subroutine like:

    sub get_todo_tasks {
        return (
            SCALAR { scalar @todo_list }      # How many?
            LIST   { @todo_list        }      # What are they?
        );
    }

    # and later...

    print "There are ", scalar(get_todo_tasks()), " tasks:\n",
            get_todo_tasks();

then you could make it much easier to interpolate calls to that
subroutine by adding:

    sub get_todo_tasks {
        return (
            SCALAR { scalar @todo_list }      # How many?
            LIST   { @todo_list        }      # What are they?

            SCALARREF { \scalar @todo_list }  # Ref to how many
            ARRAYREF  { \@todo_list        }  # Ref to them
        );
    }

    # and then...

    print "There are ${get_todo_tasks()} tasks:\n@{get_todo_tasks()}";

In fact, this behaviour is so useful that it's the default. If you
don't provide an explicit C<SCALARREF {...}> block,
Contextual::Return automatically provides an implicit one that simply
returns a reference to whatever would have been returned in scalar context.
Likewise, if no C<ARRAYREF {...}> block is specified, the module supplies one
that returns the list-context return value wrapped up in an array reference.

So you could just write:

    sub get_todo_tasks {
        return (
            SCALAR { scalar @todo_list }      # How many?
            LIST   { @todo_list        }      # What are they?
        );
    }

    # and still do this...

    print "There are ${get_todo_tasks()} tasks:\n@{get_todo_tasks()}";


=head2 Fallback contexts

As the previous sections imply, the C<BOOL {...}>, C<NUM {...}>, C<STR
{...}>, and various C<*REF {...}> blocks, are special cases of the
general C<SCALAR {...}> context block. If a subroutine is called in one
of these specialized contexts but does not use the corresponding context
block, then the more general C<SCALAR {...}> block is used instead (if
it has been specified).

So, for example:

    sub read_value_from {
        my ($fh) = @_;

        my $value = <$fh>;
        chomp $value;

        return (
            BOOL   { defined $value }
            SCALAR { $value         }
        );
    }

ensures that the C<read_value_from()> subroutine returns true in boolean
contexts if the read was successful. But, because no specific C<NUM {...}>
or C<STR {...}> return behaviours were specified, the subroutine falls back on
using its generic C<SCALAR {...}> block in all other scalar contexts.

Another way to think about this behaviour is that the various kinds of
scalar context blocks form a hierarchy:

    SCALAR
         ^
         |
         |--< BOOL
         |
         |--< NUM
         |
         `--< STR

Contextual::Return uses this hierarchical relationship to choose the most
specific context block available to handle any particular return context,
working its way up the tree from the specific type it needs, to the more
general type, if that's all that is available.

There are two slight complications to this picture. The first is that Perl
treats strings and numbers as interconvertable so the diagram (and the
Contextual::Return module) also has to allow these interconversions as a
fallback strategy:

    SCALAR
         ^
         |
         |--< BOOL
         |
         |--< NUM
         |    : ^
         |    v :
         `--< STR

The dotted lines are meant to indicate that this intraconversion is secondary
to the main hierarchical fallback. That is, in a numeric context, a C<STR
{...}> block will only be used if there is no C<NUM {...}> block I<and> no
C<SCALAR {...}> block. In other words, the generic context type is always
used in preference to string<->number conversion.

The second slight complication is that the above diagram only shows a
small part of the complete hierarchy of contexts supported by
Contextual::Return. The full fallback hierarchy (including dotted
interconversions) is:

      DEFAULT
         ^
         |
         |--< VOID
         |
         `--< NONVOID
                 ^
                 |
                 |--< VALUE <...............
                 |      ^                   :
                 |      |                   :
                 |      |--< SCALAR <.......:...
                 |      |           ^           :
                 |      |           |           :
                 |      |           |--< BOOL   :
                 |      |           |     ^     :
                 |      |           |     |     :
                 |      |           |  PUREBOOL :
                 |      |           |           :
                 |      |           |--< NUM <..:.
                 |      |           |    : ^      :
                 |      |           |    v :      :
                 |      |           `--< STR <....:..
                 |      |                           :
                 |      |                          ::
                 |      `--< LIST ................: :
                 |            : ^                   :
                 |            : :                   :
                 `--- REF     : :                   :
                       ^      : :                   :
                       |      v :                   :
                       |--< ARRAYREF                :
                       |                            :
                       |--< SCALARREF .............:
                       |
                       |--< HASHREF
                       |
                       |--< CODEREF
                       |
                       |--< GLOBREF
                       |
                       `--< OBJREF <....... METHOD

As before, each dashed arrow represents a fallback relationship. That
is, if the required context specifier isn't available, the arrows are
followed until a more generic one is found. The dotted arrows again
represent the interconversion of return values, which is
attempted only after the normal hierarchical fallback fails.

For example, if a subroutine is called in a context that expects a
scalar reference, but no C<SCALARREF {...}> block is provided, then
Contextual::Return tries the following blocks in order:

        REF {...}
    NONVOID {...}
    DEFAULT {...}
        STR {...} (automatically taking a reference to the result)
        NUM {...} (automatically taking a reference to the result)
     SCALAR {...} (automatically taking a reference to the result)
      VALUE {...} (automatically taking a reference to the result)

Likewise, in a list context, if there is no C<LIST {...}> context block, the
module tries:

       VALUE {...}
     NONVOID {...}
     DEFAULT {...}
    ARRAYREF {...} (automatically dereferencing the result)
         STR {...} (treating it as a list of one element)
         NUM {...} (treating it as a list of one element)
      SCALAR {...} (treating it as a list of one element)

The more generic context blocks are especially useful for intercepting
unexpected and undesirable call contexts. For example, to turn I<off>
the automatic scalar-ref and array-ref interpolative behaviour described
in L<Interpolative referential contexts>, you could intercept I<all>
referential contexts using a generic C<REF {...}> context block:

    sub get_todo_tasks {
        return (
            SCALAR { scalar @todo_list }      # How many?
            LIST   { @todo_list        }      # What are they?

            REF { croak q{get_todo_task() can't be used as a reference} }
        );
    }

    print 'There are ', get_todo_tasks(), '...';    # Still okay
    print "There are ${get_todo_tasks()}...";       # Throws an exception


=head2 Treating returns values as objects

Normally, when a return value is treated as an object (i.e. has a method
called on it), Contextual::Return invokes any C<OBJREF> handler that was
specified in the contextual return list, and delegates the method call to
the object returned by that handler.

However, you can also be more specific, by specifying a C<METHOD> context
handler in the contextual return list. The block of this handler is expected
to return one or more method-name/method-handler pairs, like so:

    return
        METHOD {
            get_count => sub { my $n = shift; $data[$n]{count} },
            get_items => sub { my $n = shift; $data[$n]{items} },
            clear     => sub { @data = (); },
            reset     => sub { @data = (); },
        }

Then, whenever one of the specified methods is called on the return value,
the corresponding subroutine will be called to implement it.

The method handlers must always be subroutine references, but the method-name
specifiers may be strings (as in the previous example) or they may be
specified generically, as either regexes or array references. Generic method
names are used to call the same handler for two or more distinct method names.
For example, the previous example could be simplified to:

    return
        METHOD {
            qr/get_(\w+)/     => sub { my $n = shift; $data[$n]{$1} },
            ['clear','reset'] => sub { @data = (); },
        }

A method name specified by regex will invoke the corresponding handler for any
method call request that the regex matches. A method name specified by array
ref will invoke the corresponding handler if the method requested matches any
of the elements of the array (which may themselves be strings or regexes).

When the method handler is invoked, the name of the method requested is
passed to the handler in C<$_>, and the method's argument list is passed
(as usual) via C<@_>.

Note that any methods not explicitly handled by the C<METHOD> handlers
will still be delegated to the object returned by the C<OBJREF> handler
(if it is also specified).


=head2 Deferring handlers

Because the various handlers form a hierarchy, it's possible to
implement more specific handlers by falling back on ("deferring to")
more general ones. For example, L<a C<PUREBOOL> handler|"Boolean vs Pure
Boolean contexts"> is almost always identical in its basic behaviour to
the corresponding C<BOOL> handler, except that it adds some side-effect.
For example:

    return
        PUREBOOL { $_ = $return_val; defined $return_val && $return_val > 0 }
            BOOL {                   defined $return_val && $return_val > 0 }
          SCALAR {                   $return_val;                           }

So Contextual::Return allows you to have a handler perform some action
and then defer to a more general handler to supply the actual return
value. To fall back to a more general case in this way, you simply write:

    next handler;

at the end of the handler in question, after which Contextual::Return
will find the next-most-specific handler and execute it as well. So the
previous example, could be re-written:

    return
        PUREBOOL { $_ = $return_val; next handler;        }
            BOOL { defined $return_val && $return_val > 0 }
          SCALAR { $return_val;                           }
    
Note that I<any> specific handler can defer to a more general one in
this same way. For example, you could provide consistent and
maintainable type-checking for a subroutine that returns references by
providing C<ARRAYREF>, C<HASHREF>, and C<SCALARREF> handlers that all
defer to a generic C<REF> handler, like so:

    my $retval = _get_ref();

    return
       SCALARREF { croak 'Type mismatch' if ref($retval) ne 'SCALAR';
                   next handler;
                 }
        ARRAYREF { croak 'Type mismatch' if ref($retval) ne 'ARRAY';
                   next handler;
                 }
         HASHREF { croak 'Type mismatch' if ref($retval) ne 'HASH';
                   next handler;
                 }
             REF { $retval }
        
If, at a later time, the process of returning a reference became more complex,
only the C<REF> handler would have to be updated.


=head2 Nested handlers

Another way of factoring out return behaviour is to nest more specific
handlers inside more general ones. For instance, in the final example given in
L<"Boolean vs Pure Boolean contexts">:

    sub get_data {
        my $data;
        return
            PUREBOOL {        _check_for_but_dont_get_data();   }
                BOOL { defined( $data ||= _go_and_get_data() ); }
                 REF {          $data ||= _go_and_get_data();   }
    }

you could factor out the repeated calls to C<_go_and_get_data()> like so:

    sub get_data {
        return
            PUREBOOL { _check_for_but_dont_get_data(); }
             DEFAULT {
                my $data = _go_and_get_data();

                BOOL { defined $data; }
                 REF {         $data; }
             }
    }

Here, the C<DEFAULT> handler deals with every return context except pure
boolean. Within that C<DEFAULT> handler, the data is first retrieved,
and then two "sub-handlers" deal with the ordinary boolean and
referential contexts.

Typically nested handlers are used in precisely this way: to optimize
for inexpensive special cases (such as pure boolean or integer or void
return contexts) and only do extra work for those other cases that
require it.


=head2 Failure contexts

Two of the most common ways to specify that a subroutine has failed
are to return a false value, or to throw an exception. The
Contextual::Return module provides a mechanism that allows the
subroutine writer to support I<both> of these mechanisms at the
same time, by using the C<FAIL> specifier.

A return statement of the form:

    return FAIL;

causes the surrounding subroutine to return C<undef> (i.e. false) in
boolean contexts, and to throw an exception in any other context. For example:

    use Contextual::Return;

    sub get_next_val {
        my $next_val = <>;
        return FAIL if !defined $next_val;
        chomp $next_val;
        return $next_val;
    }

If the C<return FAIL> statement is executed, it will either return false in a
boolean context:

    if (my $val = get_next_val()) {      # returns undef if no next val
        print "[$val]\n";
    }

or else throw an exception if the return value is used in any
other context:

    print get_next_val();       # throws exception if no next val

    my $next_val = get_next_val();
    print "[$next_val]\n";      # throws exception if no next val


The exception that is thrown is of the form:

    Call to main::get_next_val() failed at demo.pl line 42

but you can change that message by providing a block to the C<FAIL>, like so:

    return FAIL { "No more data" } if !defined $next_val;

in which case, the final value of the block becomes the exception message:

    No more data at demo.pl line 42


=head2 Configurable failure contexts

The default C<FAIL> behaviour--false in boolean context, fatal in all
others--works well in most situations, but violates the Platinum Rule ("Do
unto others as I<they> would have done unto them").

So it may be user-friendlier if the user of a module is allowed decide how
the module's subroutines should behave on failure. For example, one user
might prefer that failing subs always return undef; another might prefer
that they always throw an exception; a third might prefer that they
always log the problem and return a special Failure object; whilst a
fourth user might want to get back C<0> in scalar contexts, an empty list
in list contexts, and an exception everywhere else.

You could create a module that allows the user to specify all these
alternatives, like so:

    package MyModule;
    use Contextual::Return;
    use Log::StdLog;

    sub import {
        my ($package, @args) = @_;

        Contextual::Return::FAIL_WITH {
            ':false' => sub { return undef },
            ':fatal' => sub { croak @_       },
            ':filed' => sub {
                            print STDLOG 'Sub ', (caller 1)[3], ' failed';
                            return Failure->new();
                        },
            ':fussy' => sub {
                            SCALAR  { undef      }
                            LIST    { ()         }
                            DEFAULT { croak @_ }
                        },
        }, @args;
    }

This configures Contextual::Return so that, instead of the usual
false-or-fatal semantics, every C<return FAIL> within MyModule's namespace is
implemented by one of the four subroutines specified in the hash that was
passed to C<FAIL_WITH>.

Which of those four subs implements the C<FAIL> is determined by the
arguments passed after the hash (i.e. by the contents of C<@args>).
C<FAIL_WITH> walks through that list of arguments and compares
them against the keys of the hash. If a key matches an argument, the
corresponding value is used as the implementation of C<FAIL>. Note that,
if subsequent arguments also match a key, their subroutine overrides the
previously installed implementation, so only the final override has any
effect. Contextual::Return generates warnings when multiple overrides are
specified.

All of which mean that, if a user loaded the MyModule module like this:

    use MyModule qw( :fatal other args here );

then every C<FAIL> within MyModule would be reconfigured to throw an exception
in all circumstances, since the presence of the C<':fatal'> in the argument
list will cause C<FAIL_WITH> to select the hash entry whose key is C<':fatal'>.

On the other hand, if they loaded the module:

    use MyModule qw( :fussy other args here );

then each C<FAIL> within MyModule would return undef or empty list or throw an
exception, depending on context, since that's what the subroutine whose key is
C<':fussy'> does.

Many people prefer module interfaces with a C<< I<flag> => I<value> >>
format, and C<FAIL_WITH> supports this too. For example, if you
wanted your module to take a C<-fail> flag, whose associated value could
be any of C<"undefined">, C<"exception">, C<"logged">, or C<"context">,
then you could implement that simply by specifying the flag as the first
argument (i.e. I<before> the hash) like so:

    sub import {
        my $package = shift;

        Contextual::Return::FAIL_WITH -fail => {
            'undefined' => sub { return undef },
            'exception' => sub { croak @_ },
            'logged'    => sub {
                            print STDLOG 'Sub ', (caller 1)[3], ' failed';
                            return Failure->new();
                        },
            'context' => sub {
                            SCALAR  { undef      }
                            LIST    { ()         }
                            DEFAULT { croak @_ }
                        },
        }, @_;

and then load the module:

    use MyModule qw( other args here ), -fail=>'undefined';

or:

    use MyModule qw( other args here ), -fail=>'exception';

In this case, C<FAIL_WITH> scans the argument list for a pair of values: its
flag string, followed by some other selector value. Then it looks up the
selector value in the hash, and installs the corresponding subroutine as its
local C<FAIL> handler.

If this "flagged" interface is used, the user of the module can also
specify their own handler directly, by passing a subroutine reference as
the selector value instead of a string:

    use MyModule qw( other args here ), -fail=>sub{ die 'horribly'};

If this last example were used, any call to C<FAIL> within MyModule
would invoke the specified anonymous subroutine (and hence throw a
'horribly' exception).

Note that, any overriding of a C<FAIL> handler is specific to the
namespace and file from which the subroutine that calls C<FAIL_WITH> is
itself called. Since C<FAIL_WITH> is designed to be called from within a
module's C<import()> subroutine, that generally means that the C<FAIL>s
within a given module X are only overridden for the current namespace
within the particular file from module X is loaded. This means that two
separate pieces of code (in separate files or separate namespaces) can
each independently overide a module's C<FAIL> behaviour, without
interfering with each other.

=head2 Lvalue contexts

Recent versions of Perl offer (limited) support for lvalue subroutines:
subroutines that return a modifiable variable, rather than a simple constant 
value.

Contextual::Return can make it easier to create such subroutines, within the
limitations imposed by Perl itself. The limitations that Perl places on lvalue
subs are:

=over

=item 1.

The subroutine must be declared with an C<:lvalue> attribute:

    sub foo :lvalue {...}

=item 2.

The subroutine must not return via an explicit C<return>. Instead, the
last statement must evaluate to a variable, or must be a call to another
lvalue subroutine call.

    my ($foo, $baz);

    sub foo :lvalue {
        $foo;               # last statement evals to a var
    }

    sub bar :lvalue {
        foo();              # last statement is lvalue sub call
    }

    sub baz :lvalue {
        my ($arg) = @_;

        $arg > 0            # last statement evals...
            ? $baz          # ...to a var
            : bar();        # ...or to an lvalue sub call
    }

=back

Thereafter, any call to the lvalue subroutine produces a result that can be
assigned to:

    baz(0) = 42;            # same as: $baz = 42

    baz(1) = 84;            # same as:                  bar() = 84 
                            #  which is the same as:    foo() = 84
                            #   which is the same as:   $foo  = 84

Ultimately, every lvalue subroutine must return a scalar variable, which
is then used as the lvalue of the assignment (or whatever other lvalue
operation is applied to the subroutine call). Unfortunately, because the
subroutine has to return this variable I<before> the assignment
can take place, there is no way that a normal lvalue subroutine can
get access to the value that will eventually be assigned to its
return value.

This is occasionally annoying, so the Contextual::Return module offers
a solution: in addition to all the context blocks described above, it
provides three special contextual return blocks specifically for use in
lvalue subroutines: C<LVALUE>, C<RVALUE>, and C<NVALUE>.

Using these blocks you can specify what happens when an lvalue
subroutine is used in lvalue and non-lvalue (rvalue) context. For
example:

    my $verbosity_level = 1;

    # Verbosity values must be between 0 and 5...
    sub verbosity :lvalue {
        LVALUE { $verbosity_level = max(0, min($_, 5)) }
        RVALUE { $verbosity_level                      }
    }

The C<LVALUE> block is executed whenever C<verbosity> is called as an lvalue:

    verbosity() = 7;

The block has access to the value being assigned, which is passed to it
as C<$_>. So, in the above example, the assigned value of 7 would be
aliased to C<$_> within the C<LVALUE> block, would be reduced to 5 by the
"min-of-max" expression, and then assigned to C<$verbosity_level>.

(If you need to access the caller's C<$_>, it's also still available:
as C<$CALLER::_>.)

When the subroutine isn't used as an lvalue:

    print verbosity();
    
the C<RVALUE> block is executed instead and its final value returned.
Within an C<RVALUE> block you can use any of the other features of
Contextual::Return. For example:

    sub verbosity :lvalue {
        LVALUE { $verbosity_level = int max(0, min($_, 5)) }
        RVALUE {
            NUM  { $verbosity_level               }
            STR  { $description[$verbosity_level] }
            BOOL { $verbosity_level > 2           }
        }
    }

but the context sequence must be nested inside an C<RVALUE> block.

You can also specify what an lvalue subroutine should do when it is used
neither as an lvalue nor as an rvalue (i.e. in void context), by using an
C<NVALUE> block:

    sub verbosity :lvalue {
        my ($level) = @_;

        NVALUE { $verbosity_level = int max(0, min($level, 5)) }
        LVALUE { $verbosity_level = int max(0, min($_,     5)) }
        RVALUE {
            NUM  { $verbosity_level               }
            STR  { $description[$verbosity_level] }
            BOOL { $verbosity_level > 2           }
        }
    }

In this example, a call to C<verbosity()> in void context sets the verbosity
level to whatever argument is passed to the subroutine:

    verbosity(1);

Note that you I<cannot> get the same effect by nesting a C<VOID> block
within an C<RVALUE> block:

        LVALUE { $verbosity_level = int max(0, min($_, 5)) }
        RVALUE {
            NUM  { $verbosity_level               }
            STR  { $description[$verbosity_level] }
            BOOL { $verbosity_level > 2           }
            VOID { $verbosity_level = $level      }  # Wrong!
        }

That's because, in a void context the return value is never evaluated,
so it is never treated as an rvalue, which means the C<RVALUE> block
never executes.


=head2 Result blocks

Occasionally, it's convenient to calculate a return value I<before> the
end of a contextual return block. For example, you may need to clean up
external resources involved in the calculation after it's complete.
Typically, this requirement produces a slightly awkward code sequence
like this:

    return 
        VALUE {
            $db->start_work();
            my $result = $db->retrieve_query($query);
            $db->commit();
            $result;
        }

Such code sequences become considerably more awkward when you want
the return value to be context sensitive, in which case you have to
write either:

    return 
        LIST {
            $db->start_work();
            my @result = $db->retrieve_query($query);
            $db->commit();
            @result;
        }
        SCALAR {
            $db->start_work();
            my $result = $db->retrieve_query($query);
            $db->commit();
            $result;
        }

or, worse:

    return 
        VALUE {
            $db->start_work();
            my $result = LIST ? [$db->retrieve_query($query)]
                              :  $db->retrieve_query($query);
            $db->commit();
            LIST ? @{$result} : $result;
        }

To avoid these infelicities, Contextual::Return provides a second way of
setting the result of a context block; a way that doesn't require that the
result be the last statement in the block:

    return 
        LIST {
            $db->start_work();
            RESULT { $db->retrieve_query($query) };
            $db->commit();
        }
        SCALAR {
            $db->start_work();
            RESULT { $db->retrieve_query($query) };
            $db->commit();
        }

The presence of a C<RESULT> block inside a contextual return block causes
that block to return the value of the final statement of the C<RESULT>
block as the handler's return value, rather than returning the value of
the handler's own final statement. In other words, the presence of a C<RESULT>
block overrides the normal return value of a context handler.

Better still, the C<RESULT> block always evaluates its final statement
in the same context as the surrounding C<return>, so you can just write:

    return 
        VALUE {
            $db->start_work();
            RESULT { $db->retrieve_query($query) };
            $db->commit();
        }

and the C<retrieve_query()> method will be called in the appropriate context
in all cases.

A C<RESULT> block can appear anywhere inside any contextual return
block, but may not be used outside a context block. That is, this
is an error:

    if ($db->closed) {
        RESULT { undef }; # Error: not in a context block
    }
    return 
        VALUE {
            $db->start_work();
            RESULT { $db->retrieve_query($query) };
            $db->commit();
        }


=head2 Post-handler clean-up

If a subroutine uses an external resource, it's often necessary to close
or clean-up that resource after the subroutine ends...regardless of
whether the subroutine exits normally or via an exception.

Typically, this is done by encapsulating the resource in a lexically
scoped object whose destructor does the clean-up. However, if the clean-up
doesn't involve deallocation of an object (as in the C<< $db->commit() >>
example in the previous section), it can be annoying to have to create a
class and allocate a container object, merely to mediate the clean-up.

To make it easier to manage such resources, Contextual::Return supplies
a special labelled block: the C<RECOVER> block. If a C<RECOVER> block is
specified as part of a contextual return sequence, that block is
executed after any context handler, even if the context handler exits
via an exception.

So, for example, you could implement a simple commit-or-revert
policy like so:

    return 
        LIST    { $db->retrieve_all($query)  }
        SCALAR  { $db->retrieve_next($query) }
        RECOVER {
            if ($@) {
                $db->revert();
            }
            else {
                $db->commit();
            }
        }

The presence of a C<RECOVER> block also intercepts all exceptions thrown
in any other context block in the same contextual return sequence. Any
such exception is passed into the C<RECOVER> block in the usual manner:
via the C<$@> variable. The exception may be rethrown out of the
C<RECOVER> block by calling C<die>:

    return 
        LIST    { $db->retrieve_all($query) }
        DEFAULT { croak "Invalid call (not in list context)" }
        RECOVER {
            die $@ if $@;    # Propagate any exception
            $db->commit();   # Otherwise commit the changes
        }

A C<RECOVER> block can also access or replace the returned value, by
invoking a C<RESULT> block. For example:

    return 
        LIST    { attempt_to_generate_list_for(@_)  }
        SCALAR  { attempt_to_generate_count_for(@_) }
        RECOVER {
            if ($@) {                # On any exception...
                warn "Replacing return value. Previously: ", RESULT;
                RESULT { undef }     # ...return undef
            }
        }


=head2 Post-return clean-up

Occasionally it's necessary to defer the clean-up of resources until
after the return value has been used. Once again, this is usually
done by returning an object with a suitable destructor.

Using Contextual::Return you can get the same effect, by providing a
C<CLEANUP> block in the contextual return sequence:

    return 
        LIST    { $db->retrieve_all($query)  }
        SCALAR  { $db->retrieve_next($query) }
        CLEANUP { $db->commit()              }

In this example, the C<commit> method call is only performed after the
return value has been used by the caller. Note that this is quite
different from using a C<RECOVER> block, which is called as the
subroutine returns its value; a C<CLEANUP> is called when the returned
value is garbage collected.

A C<CLEANUP> block is useful for controlling resources allocated to support an
C<ACTIVE> return value. For example:

    my %file;

    # Return an active value that is always the next line from a file...
    sub readline_from {
        my ($file_name) = @_;

        # Open the file, if not already open...
        if (!$file{$file_name}) {
            open $file{$file_name}{handle}, '<', $file_name;
        }

        # Track how many active return values are using this file...
        $file{$file_name}{count}++;

        return ACTIVE
            # Evaluating the return value returns the next line...
            VALUE   { readline $file{$file_name}{handle} }

            # Once the active value is finished with, clean up the filehandle...
            CLEANUP {
                delete $file{$file_name}
                    if --$file{$file_name}{count} == 0;
            }
    }


=head2 Debugging contextual return values

Contextual return values are implemented as opaque objects (using the
"inside-out" technique). This means that passing such values to
Data::Dumper produces an uninformative output like:

    $VAR1 = bless( do{\(my $o = undef)}, 'Contextual::Return::Value' );

So the module provides two methods that allow contextual return values
to be correctly reported: either directly, or when dumped by
Data::Dumper.

To dump a contextual return value directly, call the module's C<DUMP()>
method explicitly and print the result:

    print $crv->Contextual::Return::DUMP();

This produces an output something like:

    [
     { FROM       => 'main::foo'                                       },
     { NO_HANDLER => [ 'VOID', 'CODEREF', 'HASHREF', 'GLOBREF' ]       },
     { FALLBACKS  => [ 'VALUE' ]                                       },
     { LIST       => [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]                 },
     { STR        => '<<<Throws exception: Died at demo.pl line 7.>>>' },
     { NUM        => 42                                                },
     { BOOL       => -1                                                },
     { SCALARREF  => '<<<self-reference>>>'                            },
     { ARRAYREF   => [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]                 },
    ];

The C<FROM> hash entry names the subroutine that produced the return
value. The C<NO_HANDLER> hash entry lists those contexts for which no
handler was defined (and which would therefore normally produce "can't
call" exceptions such as: C<"Can't call main::foo in VOID context">).
The C<FALLBACKS> hash entry lists any "generic" contexts such as
C<VALUE>, C<NONVOID>, C<REF>, C<DEFAULT>, etc. that the contextual
return value can also handle. After these, all the remaining hash
entries are actual contexts in which the return value could successfully
be evaluated, and the value it would produce in each of those contexts.

The Data::Dumper module also has a mechanism by which you can tell it
how to produce a similar listing automatically whenever a contextual
return value is passed to its C<Dumper> method. Data::Dumper allows you
to register a "freezer" method, that is called prior to dumping, and
which can be used to adapt an opaque object to make it dumpable.
Contextual::Return provides just such a method
(C<Contextual::Return::FREEZE()>) for you to register, like so:

    use Data::Dumper 'Dumper';
    
    local $Data::Dumper::Freezer = 'Contextual::Return::FREEZE';

    print Dumper $foo;

The output is then precisely the same as C<Contextual::Return::DUMP()>
would produce.

Note that, with both of the above dumping mechanisms, it is essential to use
the full name of the method. That is:

    print $crv->Contextual::Return::DUMP();

rather than:

    print $crv->DUMP();

This is because the shorter version is interpreted as calling the
C<DUMP()> method on the object returned by the return value's C<OBJREF>
context block (see L<"Scalar reference contexts">)

For the same reason, you must write:

    local $Data::Dumper::Freezer = 'Contextual::Return::FREEZE';

not:

    local $Data::Dumper::Freezer = 'FREEZE';


=head2 Namespace controls

By default the module exports a large number of return context markers:

    DEFAULT    REF          LAZY    
    VOID       SCALARREF    FIXED   
    NONVOID    ARRAYREF     ACTIVE  
    LIST       CODEREF      RESULT 
    SCALAR     HASHREF      RECOVER
    VALUE      GLOBREF      CLEANUP
    STR        OBJREF       RVALUE 
    NUM        METHOD       LVALUE 
    BOOL                    NVALUE 
    PUREBOOL

These are exported as subroutines, and so can conflict with existing
subroutines in your namespace, or with subroutines imported from other
modules.

Contextual::Return allows you to control which contextual return blocks are
exported into any namespace that uses the module. It also allows you to rename
blocks to avoid namespace conflicts with existing subroutines.

Both these features are controlled by passing arguments to the C<use>
statement that loads the module as follows:

=over
        
=item *

Any string passed as an argument to C<use Contextual::Return>,
exports only the block name it specifies;

=item *

Any regex passed as an argument to C<use Contextual::Return>
exports every block name it matches;

=item *

Any array ref (recursively) exports each of its elements

=item *

Any string that appears immediately after one of the above three specifiers,
and which is not itself a block name, renames the handlers exported by that
preceding specifier by filtering each handler name through C<sprintf()>

=back

That is, you can specify handlers to be exported by exact name (as a string),
by general pattern (as a regex), or collectively (in an array). And after any
of these export specifications, you can append a template in which any C<'%s'>
will be replaced by the original name of the handler. For example:

    # Selectively export specific sets of handlers...
    use Contextual::Return  qr/[NLR]VALUE/;
    use Contextual::Return  qr/.*REF/;

    # Selective export specific sets and add a suffix to each...
    use Contextual::Return  qr/[NLR]VALUE/ => '%s_CONTEXT';

    # Selective export specific sets and add a prefix to each...
    use Contextual::Return  qr/.*REF/ => 'CR_%s';

    # Export a list of handlers...
    use Contextual::Return    'NUM', 'STR', 'BOOL' ;
    use Contextual::Return qw< NUM    STR    BOOL >;
    use Contextual::Return   ['NUM', 'STR', 'BOOL'];

    # Export a list of handlers, renaming them individually...
    use Contextual::Return  NUM => 'NUMERIC', STR => 'TEXT', BOOL => 'CR_%s';
            
    # Export a list of handlers, renaming them collectively...
    use Contextual::Return  ['NUM', 'STR', 'BOOL'] => '%s_CONTEXT';

    # Mixed exports and renames...
    use Contextual::Return (
        STR => 'TEXT',
        ['NUM', 'BOOL'] => 'CR_%s',
        ['LIST', 'SCALAR', 'VOID', qr/^[NLR]VALUE/] => '%s_CONTEXT',
    );



=head1 INTERFACE 

=head2 Context tests

=over 

=item C<< LIST() >>

Returns true if the current subroutine was called in list context.
A cleaner way of writing: C<< wantarray() >>

=item C<< SCALAR() >>

Returns true if the current subroutine was called in scalar context.
A cleaner way of writing: C<< defined wantarray() && ! wantarray() >>


=item C<< VOID() >>

Returns true if the current subroutine was called in void context.
A cleaner way of writing: C<< !defined wantarray() >>

=item C<< NONVOID() >>

Returns true if the current subroutine was called in list or scalar context.
A cleaner way of writing: C<< defined wantarray() >>

=back

=head2 Standard contexts

=over 

=item C<< LIST {...} >>

The block specifies what the context sequence should evaluate to when
called in list context.

=item C<< SCALAR {...} >>

The block specifies what the context sequence should evaluate to in
scalar contexts, unless some more-specific specifier scalar context specifier
(see below) also occurs in the same context sequence.

=item C<< VOID {...} >>

The block specifies what the context sequence should do when
called in void context.

=back

=head2 Scalar value contexts

=over

=item C<< BOOL {...} >>

The block specifies what the context sequence should evaluate to when
treated as a boolean value.

=item C<< NUM {...} >>

The block specifies what the context sequence should evaluate to when
treated as a numeric value.

=item C<< STR {...} >>

The block specifies what the context sequence should evaluate to when
treated as a string value.

=item C<< LAZY {...} >>

Another name for C<SCALAR {...}>. Usefully self-documenting when the primary
purpose of the contextual return is to defer evaluation of the return value
until it's actually required.

=back

=head2 Scalar reference contexts

=over

=item C<< SCALARREF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference to a scalar.

=item C<< ARRAYREF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference to an array.

=item C<< HASHREF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference to a hash.

Note that a common error here is to write:

HASHREF { a=>1, b=>2, c=>3 }

The curly braces there are a block, not a hash constructor, so the block
doesn't return a hash reference and the interpreter throws an exception.
What's needed is:

HASHREF { {a=>1, b=>2, c=>3} }

in which the inner braces I<are> a hash constructor.

=item C<< CODEREF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference to a subroutine.

=item C<< GLOBREF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference to a typeglob.

=item C<< OBJREF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference to an object.

=item C<< METHOD {...} >>

The block can be used to specify particular handlers for specific method calls
when the return value is treated as an object reference.
It should return a list of methodname/methodbody pairs. Each method name can
be specified as a string, a regex, or an array of strings or regexes. The
method bodies must be specified as subroutine references (usually anonymous
subs). The first method name that matches the actual method call selects the
corresponding handler, which is then called.

=back

=head2 Generic contexts

=over

=item C<< VALUE {...} >>

The block specifies what the context sequence should evaluate to when
treated as a non-referential value (as a boolean, numeric, string,
scalar, or list). Only used if there is no more-specific value context
specifier in the context sequence.

=item C<< REF {...} >>

The block specifies what the context sequence should evaluate to when
treated as a reference of any kind. Only used if there is no
more-specific referential context specifier in the context sequence.

=item C<< NONVOID {...} >>

The block specifies what the context sequence should evaluate to when
used in a non-void context of any kind. Only used if there is no
more-specific context specifier in the context sequence.

=item C<< DEFAULT {...} >>

The block specifies what the context sequence should evaluate to when
used in a void or non-void context of any kind. Only used if there is no
more-specific context specifier in the context sequence.

=back

=head2 Failure context

=over

=item C<< FAIL >>

This block is executed unconditionally and is used to indicate failure. In a
Boolean context it return false. In all other contexts it throws an exception
consisting of the final evaluated value of the block.

That is, using C<FAIL>:

return
FAIL { "Could not defenestrate the widget" }

is exactly equivalent to writing:

return
BOOL { 0 }
DEFAULT { croak "Could not defenestrate the widget" }

except that the reporting of errors is a little smarter under C<FAIL>.

If C<FAIL> is called without specifying a block:

return FAIL;

it is equivalent to:

return FAIL { croak "Call to <subname> failed" }

(where C<< <subname> >> is replaced with the name of the surrounding
subroutine).

Note that, because C<FAIL> implicitly covers every possible return
context, it cannot be chained with other context specifiers.

=item C<< Contextual::Return::FAIL_WITH >>

This subroutine is not exported, but may be called directly to reconfigure
C<FAIL> behaviour in the caller's namespace. 

The subroutine is called with an optional string (the I<flag>), followed
by a mandatory hash reference (the I<configurations hash>), followed by a
list of zero-or-more strings (the I<selector list>). The values of the
configurations hash must all be subroutine references.

If the optional flag is specified, C<FAIL_WITH> searches the selector
list looking for that string, then uses the I<following> item in the
selector list as its I<selector value>. If that selector value is a
string, C<FAIL_WITH> looks up that key in the hash, and installs the
corresponding subroutine as the namespace's C<FAIL> handler (an
exception is thrown if the selector string is not a valid key of the
configurations hash). If the selector value is a subroutine reference,
C<FAIL_WITH> installs that subroutine as the C<FAIL> handler.

If the optional flag is I<not> specified, C<FAIL_WITH> searches the
entire selector list looking for the last element that matches any
key in the configurations hash. It then looks up that key in the
hash, and installs the corresponding subroutine as the namespace's
C<FAIL> handler.

See L<Configurable failure contexts> for examples of using this feature.

=back

=head2 Lvalue contexts

=over

=item C<< LVALUE >>

This block is executed when the result of an C<:lvalue> subroutine is assigned
to. The assigned value is passed to the block as C<$_>. To access the caller's
C<$_> value, use C<$CALLER::_>.

=item C<< RVALUE >>

This block is executed when the result of an C<:lvalue> subroutine is used
as an rvalue. The final value that is evaluated in the block becomes the
rvalue.

=item C<< NVALUE >>

This block is executed when an C<:lvalue> subroutine is evaluated in void
context.

=back

=head2 Explicit result blocks

=over 

=item C<< RESULT >> 

This block may only appear inside a context handler block. It causes the
surrounding handler to return the final value of the C<RESULT>'s block,
rather than the final value of the handler's own block. This override occurs
regardless of the location to the C<RESULT> block within the handler.

If called without a trailing C<{...}>, it simply returns the current result
value in scalar contexts, or the list of result values in list context.

=back

=head2 Recovery blocks

=over 

=item C<< RECOVER >> 

If present in a context return sequence, this block grabs control after
any context handler returns or exits via an exception. If an exception
was thrown it is passed to the C<RECOVER> block via the C<$@> variable.

=back

=head2 Clean-up blocks

=over 

=item C<< CLEANUP >> 

If present in a context return sequence, this block grabs control when
a return value is garbage collected.

=back


=head2 Modifiers

=over

=item C<< FIXED >>

This specifies that the scalar value will only be evaluated once, the
first time it is used, and that the value will then morph into that
evaluated value.

=item C<< ACTIVE >>

This specifies that the scalar value's originating block will be re-
evaluated every time the return value is used.

=back

=head2 Debugging support

=over

=item C<< $crv->Contextual::Return::DUMP() >>

Dump a representation of the return value in all viable contexts

=item C<< local $Data::Dumper::Freezer = 'Contextual::Return::FREEZE' >>

Configure Data::Dumper to correctly dump a representation of the
return value.

=back


=head1 DIAGNOSTICS

=over 

=item C<Can't use %s as export specifier>

In your C<use Contextual::Return> statement you specified something (such as a
hash or coderef) that can't be used to select what the module exports. Make
sure the list of selectors includes only strings, regexes, or references to
arrays of strings or regexes.


=item C<use Contextual::Return qr{%s} didn't export anything>

In your C<use Contextual::Return> statement you specified a regex to select
which handlers to support, but the regex didn't select any handlers. Check
that the regex you're using actually does match at least one of the names of
the modules many handlers.


=item C<Can't export %s: no such handler>

In your C<use Contextual::Return> statement you specified a string as the
name of a context handler to be exported, but the module doesn't export a
handler of that name. Check the spelling for the requested export.


=item C<Can't call %s in %s context>

The subroutine you called uses a contextual return, but doesn't specify what
to return in the particular context in which you called it. You either need to
change the context in which you're calling the subroutine, or else add a
context block corresponding to the offending context (or perhaps a
C<DEFAULT {...}> block).


=item C<%s can't return a %s reference">

You called the subroutine in a context that expected to get back a
reference of some kind but the subroutine didn't specify the
corresponding C<SCALARREF>, C<ARRAYREF>, C<HASHREF>, C<CODEREF>,
C<GLOBREF>, or generic C<REF>, C<NONVOID>, or C<DEFAULT> handlers.
You need to specify the appropriate one of these handlers in the subroutine.


=item C<Can't call method '%s' on %s value returned by %s">

You called the subroutine and then tried to call a method on the return
value, but the subroutine returned a classname or object that doesn't
have that method. This probably means that the subroutine didn't return
the classname or object you expected. Or perhaps you need to specify
an C<OBJREF {...}> context block.


=item C<Can't install two %s handlers>

You attempted to specify two context blocks of the same name in the same
return context, which is ambiguous. For example:

    sub foo: lvalue {
        LVALUE { $foo = $_ }
        RVALUE { $foo }
        LVALUE { $foo = substr($_,1,10) }
    }

or:

    sub bar {
        return
            BOOL { 0 }
            NUM  { 1 }
            STR  { "two" }
            BOOL { 1 };
    }

Did you cut-and-paste wrongly, or mislabel one of the blocks?


=item C<Expected a %s block after the %s block but found instead: %s>

If you specify any of C<LVALUE>, C<RVALUE>, or C<NVALUE>, then you can only
specify C<LVALUE>, C<RVALUE>, or C<NVALUE> blocks in the same return context.
If you need to specify other contexts (like C<BOOL>, or C<STR>, or C<REF>,
etc.), put them inside an C<RVALUE> block. See L<Lvalue contexts> for an
example.


=item C<Call to %s failed at %s>

This is the default exception that a C<FAIL> throws in a non-scalar
context. Which means that the subroutine you called has signalled
failure by throwing an exception, and you didn't catch that exception.
You should either put the call in an C<eval {...}> block or else call the
subroutine in boolean context instead.


=item C<Call to %s failed at %s. Attempted to use failure value at %s>

This is the default exception that a C<FAIL> throws when a failure value
is captured in a scalar variable and later used in a non-boolean
context. That means that the subroutine you called must have failed, and
you didn't check the return value for that failure, so when you tried to
use that invalid value it killed your program. You should either put the
original call in an C<eval {...}> or else test the return value in a
boolean context and avoid using it if it's false.


=item C<Usage: FAIL_WITH $flag_opt, \%selector, @args>

The C<FAIL_WITH> subroutine expects an optional flag, followed by a reference
to a configuration hash, followed by a list or selector arguments. You gave it
something else. See L<Configurable Failure Contexts>.


=item C<Selector values must be sub refs>

You passed a configuration hash to C<FAIL_WITH> that specified non-
subroutines as possible C<FAIL> handlers. Since non-subroutines can't
possibly be handlers, maybe you forgot the C<sub> keyword somewhere?


=item C<Invalid option: %s => %s>

The C<FAIL_WITH> subroutine was passed a flag/selector pair, but the selector
was not one of those allowed by the configuration hash.


=item C<FAIL handler for package %s redefined>

A warning that the C<FAIL> handler for a particular package was
reconfigured more than once. Typically that's because the module was
loaded in two places with difference configurations specified. You can't
reasonably expect two different sets of behaviours from the one module within
the one namespace.

=back


=head1 CONFIGURATION AND ENVIRONMENT

Contextual::Return requires no configuration files or environment variables.


=head1 DEPENDENCIES

Requires version.pm and Want.pm.


=head1 INCOMPATIBILITIES

C<LVALUE>, C<RVALUE>, and C<NVALUE> do not work correctly under the Perl
debugger. This seems to be because the debugger injects code to capture
the return values from subroutines, which interferes destructively with 
the optional final arguments that allow C<LVALUE>, C<RVALUE>, and C<NVALUE>
to cascade within a single return.


=head1 BUGS AND LIMITATIONS

No bugs have been reported.


=head1 AUTHOR

Damian Conway  C<< <DCONWAY@cpan.org> >>


=head1 LICENCE AND COPYRIGHT

Copyright (c) 2005-2006, Damian Conway C<< <DCONWAY@cpan.org> >>. All rights reserved.

This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.


=head1 DISCLAIMER OF WARRANTY

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL,
OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE
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