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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 2.1 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 (ice-9 syncase)
+ #: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)))))