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;;; srfi-39.scm --- Parameter objects
;; Copyright (C) 2004, 2005, 2006, 2008 Free Software Foundation, Inc.
;;
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public
;; License as published by the Free Software Foundation; either
;; version 3 of the License, or (at your option) any later version.
;;
;; This library is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
;;; Author: Jose Antonio Ortega Ruiz <jao@gnu.org>
;;; Date: 2004-05-05
;;; Commentary:
;; This is an implementation of SRFI-39 (Parameter objects).
;;
;; The implementation is based on Guile's fluid objects, and is, therefore,
;; thread-safe (parameters are thread-local).
;;
;; In addition to the forms defined in SRFI-39 (`make-parameter',
;; `parameterize'), a new procedure `with-parameters*' is provided.
;; This procedures is analogous to `with-fluids*' but taking as first
;; argument a list of parameter objects instead of a list of fluids.
;;
;;; Code:
(define-module (srfi srfi-39)
#:use-module (srfi srfi-16)
#:export (make-parameter)
#:export-syntax (parameterize)
;; helper procedure not in srfi-39.
#:export (with-parameters*)
#:replace (current-input-port current-output-port current-error-port))
;; Make 'srfi-39 available as a feature identifiere to `cond-expand'.
;;
(cond-expand-provide (current-module) '(srfi-39))
(define make-parameter
(case-lambda
((val) (make-parameter/helper val (lambda (x) x)))
((val conv) (make-parameter/helper val conv))))
(define get-fluid-tag (lambda () 'get-fluid)) ;; arbitrary unique (as per eq?) value
(define get-conv-tag (lambda () 'get-conv)) ;; arbitrary unique (as per eq?) value
(define (make-parameter/helper val conv)
(let ((value (make-fluid))
(conv conv))
(begin
(fluid-set! value (conv val))
(lambda new-value
(cond
((null? new-value) (fluid-ref value))
((eq? (car new-value) get-fluid-tag) value)
((eq? (car new-value) get-conv-tag) conv)
((null? (cdr new-value)) (fluid-set! value (conv (car new-value))))
(else (error "make-parameter expects 0 or 1 arguments" new-value)))))))
(define-syntax parameterize
(syntax-rules ()
((_ ((?param ?value) ...) ?body ...)
(with-parameters* (list ?param ...)
(list ?value ...)
(lambda () ?body ...)))))
(define (current-input-port . new-value)
(if (null? new-value)
((@ (guile) current-input-port))
(apply set-current-input-port new-value)))
(define (current-output-port . new-value)
(if (null? new-value)
((@ (guile) current-output-port))
(apply set-current-output-port new-value)))
(define (current-error-port . new-value)
(if (null? new-value)
((@ (guile) current-error-port))
(apply set-current-error-port new-value)))
(define port-list
(list current-input-port current-output-port current-error-port))
;; There are no fluids behind current-input-port etc, so those parameter
;; objects are picked out of the list and handled separately with a
;; dynamic-wind to swap their values to and from a location (the "value"
;; variable in the swapper procedure "let").
;;
;; current-input-port etc are already per-dynamic-root, so this arrangement
;; works the same as a fluid. Perhaps they could become fluids for ease of
;; implementation here.
;;
;; Notice the use of a param local variable for the swapper procedure. It
;; ensures any application changes to the PARAMS list won't affect the
;; winding.
;;
(define (with-parameters* params values thunk)
(let more ((params params)
(values values)
(fluids '()) ;; fluids from each of PARAMS
(convs '()) ;; VALUES with conversion proc applied
(swapper noop)) ;; wind/unwind procedure for ports handling
(if (null? params)
(if (eq? noop swapper)
(with-fluids* fluids convs thunk)
(dynamic-wind
swapper
(lambda ()
(with-fluids* fluids convs thunk))
swapper))
(if (memq (car params) port-list)
(more (cdr params) (cdr values)
fluids convs
(let ((param (car params))
(value (car values))
(prev-swapper swapper))
(lambda ()
(set! value (param value))
(prev-swapper))))
(more (cdr params) (cdr values)
(cons ((car params) get-fluid-tag) fluids)
(cons (((car params) get-conv-tag) (car values)) convs)
swapper)))))
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