messages

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This library implements messages as special, sometimes parametrized, generics of arity one as well as protocols. For the sake of simplicity, all method-trees, which are in fact lists, have a sentinel with selector any?? which branches to the default behaviour, calling the error routine.

Protocols are simply lists of messages. To check, if a protocol adopts an object, one can inspect the selectors of all method-trees: besides any?? there must be another selector which accepts the object.

The three fundamental macros are

 (define-message ((Msg arg . args) obj))
 (define-message (Msg obj))

for parametrized or simple messages respectively,

 (define-protocol NAME Msg|(Msg arg . args) ....)

and

 (define-method (Name (x x??)) body ....)

the latter being reexported from the generics library. Note, that body must be a lambda expression with arguments arg . args in case of a parametrized message.

The module messages

messages sym ..procedure: documentation procedure: returns the list of exported symbols when called as thunk or the documentation of its argument sym.

define-message

<macro>(define-message ((Msg arg . args) obj))</macros> <macro>(define-message (Msg obj))</macro>

defines a message named Msg, which might be parametrized. Messages are generic functions of arity one, which, when parametrized, are called differently than generics.

define-method

(define-method (Msg (x x??)) body ....)syntax: inserts an anonymous method constructed from argument x, selector x?? and body .... into the method tree of the generic function Msg at the position determined by selector's parents. The body must be a lambda expression in case of a message with parameters. Reexported from generics.

message?

message? xprprocedure: type predicate

message-variadic?

message-variadic? Msgprocedure: is the message function Msg variadic?

message-arity

message-arity Msgprocedure: returns the arity of the message Msg

define-protocol

(define-protocol NAME Msg ....)syntax: where Msg is either a parametrized message of the form (Name arg ....) or a parameterless message Name. Defines a protocol, NAME, consisting of a list of messages.

protocol-adopts?

protocol-adopts? NAME objprocedure: can each message of protocol NAME operate on obj?

protocol-docs

protocol-docs NAMEprocedure: returns a readable list of the messages defined in protocol NAME

Requirements

generics simple-cells

Usage

(use messages generics simple-cells)

Examples

;;; Sequences
;;; ---------
(define-protocol SEQ (Drop k) (Take k) (At k))
(define-method (Drop (seq string??))
               (lambda (k) (substring seq k)))
(define-method (Drop (seq vector??))
               (lambda (k) (subvector seq k)))
(define-method (Drop (seq list??))
               (lambda (k) (list-tail seq k)))
(define-method (Take (seq string??))
               (lambda (k) (substring seq 0 k)))
(define-method (Take (seq vector??))
               (lambda (k) (subvector seq 0 k)))
(define-method (Take (seq list??))
               (lambda (k)
                 (let loop ((n 0) (tail seq) (head '()))
                   (if (fx= k n)
                     (reverse head)
                     (loop (1+ n)
                           (cdr tail)
                           (cons (car tail) head))))))
(define-method (At (seq string??))
               (lambda (k) (string-ref seq k)))
(define-method (At (seq vector??))
               (lambda (k) (vector-ref seq k)))
(define-method (At (seq list??))
               (lambda (k) (list-ref seq k)))

((At 2) '(0 1 2 3 4)) ; -> 2
((Drop 1) "01234") ; -> "1234"
((Take 3) #(0 1 2 3 4)) ; -> #(0 1 2)

;;; Stacks
;;; -------
(define-protocol STACK (Push! arg) Pop! Top)
(define-selector stack?? list?? cell?)
(define-method (Push! (obj stack??))
               (lambda (arg)
                 (let ((old (obj)))
                   (obj (cons arg old))
                   old)))
(define-method (Top (obj stack??))
               (if (null? (obj))
                 (gensym 'stack-empty)
                 (car (obj))))
(define-method (Pop! (obj stack??))
               (if (null? (obj))
                 (gensym 'stack-empty)
                 (let ((old (obj)))
                   (obj (cdr old))
                   old)))
(define stk (cell '() list?))
((Push! 0) stk) ; -> '()
((Push! 1) stk) ; -> '(0)
(Top stk) ; -> 1
((Push! 2) stk) ; -> '(1 0)
(Top stk) ; -> 2
(Pop! stk) ; -> '(2 1 0)
(Top stk) ; -> 1

Copyright (c) 2018, Juergen Lorenz (ju (at) jugilo (dot) de)
All rights reserved.

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Version History

0.1: initial import

chickadee » messages

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