A while ago, I finished a SRFI-45 compliant force/delay/eager
implementation for Guile, the core of which is more or less a
translation of the SRFI reference code.
(I started this in part with an eye toward lazy process streams,
i.e. "find -printf ..." -> lazy-stream-of-lines)
The implementation works (it passes all of the tests they provide),
and I believe could completely replace Guile's existing eval.c
force/delay, but I wanted to know if anyone knew of a reason it
shouldn't.
One notable difference is that this implementation is pure Scheme
code (I have the eval.c force/delay commented out here).
Below is what the current code looks like (just to give you a rough
idea). Of course we would s/newforce/force/g and s/newdelay/delay/g
before actually incorporating it.
(define-module (ice-9 srfi-45)
:use-module (ice-9 threads))
(export newforce)
(export-syntax newdelay)
(export eager)
(export-syntax lazy)
(define (print-promise promise port)
(with-mutex (promise-mutex promise)
(display "#<promise " port)
(display (promise-kind promise))
(display " (value ")
(write (promise-value promise))
(display ")>")))
(define promise-type
(make-record-type "promise" '(mutex data) print-promise))
(define promise-data (record-accessor promise-type 'data))
(define set-promise-data! (record-modifier promise-type 'data))
(define promise-mutex (record-accessor promise-type 'mutex))
(define make-promise
(let ((constructor (record-constructor promise-type '(mutex data))))
(lambda (kind val)
(constructor (make-recursive-mutex) (cons kind val)))))
(define (promise-kind p) (car (promise-data p)))
(define (set-promise-kind! p v) (set-car! (promise-data p) v))
(define (promise-value p) (cdr (promise-data p)))
(define (set-promise-value! p v) (set-cdr! (promise-data p) v))
;; need to finish this one...
;;(define promise?
;; "Return true if @var{obj} is a promise, i.e. a delayed
computation\n(@pxref{Delayed evaluation,,,r5rs.info,The Revised^5 Report on
Scheme})."
;; (record-predicate promise-type))
(define-macro (newdelay exp)
`(lazy (eager ,exp)))
(define (eager exp)
(make-promise 'eager exp))
(define-macro (lazy exp)
`((@@ (ice-9 srfi-45) make-promise) 'lazy (lambda () ,exp)))
(define (newforce promise)
(define (force-aux p)
(case (promise-kind p)
((eager) (promise-value promise))
((lazy)
(let* ((promise* ((promise-value p))))
(if (not (eq? 'eager (promise-kind p)))
(begin
(set-promise-kind! p (promise-kind promise*))
(set-promise-value! p (promise-value promise*))
(set-promise-data! promise* (promise-data p))
)
)
(force-aux p)))))
(with-mutex (promise-mutex promise)
(force-aux promise)))
--
Rob Browning
rlb @defaultvalue.org and @debian.org; previously @cs.utexas.edu
GPG starting 2002-11-03 = 14DD 432F AE39 534D B592 F9A0 25C8 D377 8C7E 73A4
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