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Semantical error recovery once again requires judgment on the part of the
grammar-writer. For example, an assignment expression may be syntactically
defined as
verb(
expr '=' expr
)
The left-hand side must be a so-called em(lvalue). An em(lvalue) is simply
an addressable location, like a variable's identifier, a dereferenced pointer
expression or some other address-expression. The right-hand side is a
so-called em(rvalue): this may be any value: any expression will do.
A rule like the above leaves room for many different semantical errors:
itemization(
it() Since the rule states tt(expr) at its left-hand side, em(any)
expression will be accepted by the parser. E.g.,
verb(
3 = 12
)
So, the action associated with this rule should em(check) whether the
left-hand side is actually an lvalue. If not, a em(semantical) error should be
reported;
it() In a typed language (like bf(C++)), not all assignments are
possible. E.g., it is not acceptable to assign a bf(std:string) value
to a bf(double) variable. When conflicting types are used, a em(semantical)
error should be reported;
it() In a language requiring variables to be defined or declared before
they are used (like bf(C++)) the parser should check whether a variable is
actually defined or declared when it is used in an expression. If not, a
em(semantical) error should be reported
)
A parser that should be able to detect semantic errors will normally use a
counter counting the number of semantic errors, e.g., tt(size_t
d_nSemanticErrors). It may be possible to test this counter's value once the
input has been parsed, calling tt(ABORT()) (see section ref(PRIVMEM)) if the
counter isn't zero anymore. When the grammar's start symbol itself has
multiple alternatives, it is probably easiest to augment the grammar with an
additional rule, becoming the augmented grammar's start symbol which simply
calls the former start symbol. For example, if tt(input) was the name of the
original start-symbol, augment the grammar as follows to ensure a
bf(PARSE_ABORT) return value of the tt(parse()) member when either syntactic
or semantical errors were detected:
verb(
semantic_input: // new start-symbol
input
{
if (d_nSemanticErrors) // return PARSE_ABORT
ABORT(); // on semantic errors too.
}
)
Returning from the parser's tt(parse()) member the number of syntactic
and semantical errors could then be printed, whereupon the program might
terminate.
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