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  INTERFACE  cf_state_gadget.mli

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(** Monadic composition of complex stream processors.  An experimental
    interface for constructing interactive functional systems in a single
    thread of control.
*)

(** {6 Overview}

    This module implements a marginally more general version of the Gadget
    system described in Chapter 30 of Magnus Carlsson's and Thomas Hallgren's
    joint {{:http://www.cs.chalmers.se/~hallgren/Thesis/}Ph.D. thesis}.
    
    In the context of this module, a "gadget" is a monad that evaluates into
    a {!Cf_flow} object, capable of alternately reading from a source of input
    values and writing to a sink of output values.  The continuation monad is
    specialized over an abstract "process" monad type, and a scheduler handles
    the calls and jumps between multiple simultaneous processes communicating
    with one another over a very lightweight message passing abstraction called
    a "wire".
    
    The abstract process monad is a kind of state-continuation monad for
    operations over the internal {!Cf_flow} value.  The operations it supports
    are lifted into the gadget monad, as are briefly sumamrized as follows:
    
    {ul
        {- {i start}: launch a new process in the scheduler.}
        {- {i wire}: create a new message wire.}
        {- {i put}: send a message on a wire.}
        {- {i get}: create a gate for receiving messages on a wire.}
        {- {i guard}: receive a message from one of several gates.}
        {- {i load}: load the current process state.}
        {- {i store}: store a new value of the process state.}
        {- {i read}: read a new value from the external input.}
        {- {i write}: write a new value to the external output.}
    }
    
    A wire is logically composed of a receiver and a transmitter, with weak
    mutual references between them.  When either end of the wire is reclaimed
    by the memory allocator, the other end is automatically rendered into a
    null wire, i.e. receivers never get messages and transmitters put messages
    by discarding them.
    
    A pair of classes are provided to represent the receiver and the
    transmitter on a wire.  Objects of the [rx] class define a [get] method for
    creating a "gate" that can receive a message.  Objects of the [tx] class
    define a [put] method for transmitting a message.  Both objects can be
    constructed with a wire object, and a convenience operators are defined for
    creating a new wire and construction a pair of associated [rx] and [tx]
    objects.
    
    Each process contains an encapsulated state, initialized to a value when
    the process is started.  As a process receives and transmits messages, it
    can easily manipulate this encapsulated state.  When a process has no more
    messages to receive or transmit, the scheduler reclaims its resources and
    the final state is discarded.
    
    Any process may read from the internal input stream or write to the
    external output stream.  Conventionally, it is often simpler to define a
    a reader process and a writer process to localize these effects.
    
    {b Note}: see Magnus Carlsson's and Thomas Hallgren's joint
    {{:http://www.cs.chalmers.se/~hallgren/Thesis/}Ph.D. thesis} for a complete
    dissertation on the nature of the system of concepts behind this module.
*)

(** {6 Types} *)

(** An functionally compositional unit of work in a gadget.  It encapsulates
    the state-continuation monad for a work loop.
*)
type ('s, 'i, 'o) work

(** A gate for receiving messages in a process of type [('s, 'i, 'o) work]
    using the [guard] function.
*)
type ('s, 'i, 'o) gate

(** An object capable of delivering messages of type ['x] from a sender to a
    a receiver in a [('s, 'i, 'o) work] continuation.
*)
type ('x, 'i, 'o) wire

(** A guard for receiving a message from one or more sources. *)
type ('s, 'i, 'o, 'a) guard = (('s, 'i, 'o) gate, 'a) Cf_cmonad.t

(** A continuation monad parameterized by process type. *)
type ('s, 'i, 'o, 'a) t = (('s, 'i, 'o) work, 'a) Cf_cmonad.t

(** {6 Functions} *)

(** Use [eval y s] to obtain a new flow by evaluating the gadget monad [y] with
    a state initializer of [a].
*)
val eval: ('s, 'i, 'o, unit) t -> 's -> ('i, 'o) Cf_flow.t

(** Bind the result of [start y s] to start a new process evaluating the gadget
    [y] with a state initializer [s].
*)
val start: ('s0, 'i, 'o, unit) t -> 's0 -> ('s1, 'i, 'o, unit) t

(** Use [guard m] to receive the next message guarded by [m].  Control will
    pass to the continuation of the first gate in the guard to receive a
    message.  If no gates in the guard are able to receive a message, i.e. the
    guard is either empty or all of the gates are on wires that have no
    transmitters anymore, then control is returned to the scheduler.
*)
val guard: ('s, 'i, 'o, unit) guard -> ('s, 'i, 'o, 'a) t

(** Use [abort] to abort processing and return to the scheduler.  Control will
    not be passed to any continuation bound to the result.
*)
val abort: ('s, 'i, 'o, 'a) t

(** Use [wire] to return a new wire for carrying messages of type ['x]. *)
val wire: ('s, 'i, 'o, ('x, 'i, 'o) wire) t

(** Use [wirepair] to return a pair of new wires for carrying messages of type
    ['x] and ['y].
*)
val wirepair: ('s, 'i, 'o, ('x, 'i, 'o) wire * ('y, 'i, 'o) wire) t

(** Use [null] to construct a [rx] object that produces gates that never
    receive any messages, and a [tx] object that discards every message
    transmitted without deliver it.  This object can be useful for default
    arguments to some gadget functions.
*)
val null: ('x, 'i, 'o) wire

(** Bind [read] to get the next input value from the external stream. *)
val read: ('s, 'i, 'o, 'i) t

(** Bind the result of [write obj] to put the next output value into the
    external stream.
*)
val write: 'o -> ('s, 'i, 'o, unit) t

(** Bind [load] to get the current state encapsulated in the process. *)
val load: ('s, 'i, 'o, 's) t

(** Bind the result of [store obj] to store the state [obj] as the encapsulated
    state for the current monad.
*)
val store: 's -> ('s, 'i, 'o, unit) t

(** Bind the result of [modify f] to apply [f] to the current encapsulated
    state of the process and store the resulting new state.
*)
val modify: ('s -> 's) -> ('s, 'i, 'o, unit) t

(** {6 Classes} *)

(** The class type of connector objects. *)
class type connector =
    object
        (** Returns [true] if the other end of the wire has not yet been
            reclaimed by the garbage collector.
        *)
        method check: bool
        
        (** Returns a string representation of the wire end identifier. *)
        method id: string
    end

(** The class of receiver objects. *)
class ['x, 'i, 'o] rx:
    ('x, 'i, 'o) wire -> (** A wire carrying messages of type ['x]. *)
    object
        inherit connector
        
        (** Use [rx#get f] to produce a guard that receives a message on the
            associated wire by applying the function [f] to it.
        *)
        method get:
            's. ('x -> ('s, 'i, 'o, unit) t) -> ('s, 'i, 'o, unit) guard
    end

(** The class of transmitter objects. *)
class ['x, 'i, 'o] tx:
    ('x, 'i, 'o) wire -> (** A wire carrying messages of type ['x]. *)
    object
        inherit connector
        
        (** Use [tx#put obj] to schedule the message obj for deliver on the
            associated wire.
        *)
        method put: 's. 'x -> ('s, 'i, 'o, unit) t
    end

(** {6 Miscellaneous} *)

(** Use [simplex] to construct a new maching pair of [rx] and [tx] objects.*)
val simplex: ('s, 'i, 'o, ('x, 'i, 'o) rx * ('x, 'i, 'o) tx) t

(** A pair of convenience types for representing each end of a bundle of two
    wires used for duplex communication.
*)
type ('x, 'y, 'i, 'o) pad = ('x, 'i, 'o) rx * ('y, 'i, 'o) tx
type ('x, 'y, 'i, 'o) fix = ('y, 'i, 'o) rx * ('x, 'i, 'o) tx

(** Use [duplex] to construct a new duplex communication channel, composed of
    two wires each in opposite flow.  A matching head and tail of the channel
    is returned.
*)
val duplex: ('s, 'i, 'o, ('x, 'y, 'i, 'o) pad * ('x, 'y, 'i, 'o) fix) t

(** Use [wrap rx tx w] to start a new process that wraps the flow [w], so that
    it reads output from the flow (copying it to [tx] object) and writes input
    to the flow (copying it from the [rx] object).
*)
val wrap:
    ('x, 'i, 'o) #rx -> ('y, 'i, 'o) #tx -> ('x, 'y) Cf_flow.t ->
    ('s, 'i, 'o, unit) t

(*--- End of File [ cf_state_gadget.mli ] ---*)