;; Copyright (c) 2009 Derick Eddington
;;
;; Permission is hereby granted, free of charge, to any person obtaining a
;; copy of this software and associated documentation files (the "Software"),
;; to deal in the Software without restriction, including without limitation
;; the rights to use, copy, modify, merge, publish, distribute, sublicense,
;; and/or sell copies of the Software, and to permit persons to whom the
;; Software is furnished to do so, subject to the following conditions:
;;
;; The above copyright notice and this permission notice shall be included in
;; all copies or substantial portions of the Software.
;;
;; Except as contained in this notice, the name(s) of the above copyright
;; holders shall not be used in advertising or otherwise to promote the sale,
;; use or other dealings in this Software without prior written authorization.
;;
;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
;; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
;; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
;; THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
;; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
;; FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
;; DEALINGS IN THE SOFTWARE.

;; Fall-back library in case the host Scheme system does not provide SRFI-39.

#!r6rs
(library (srfi :39 parameters)
  (export
    make-parameter 
    parameterize)
  (import
    (rnrs))

  (define make-parameter
    (case-lambda
      [(val) (make-parameter val values)]
      [(val guard)
       (unless (procedure? guard)
         (assertion-violation 'make-parameter "not a procedure" guard))
       (let ([p (case-lambda
                  [() val]
                  [(x) (set! val (guard x))])])
         (p val)
         p)]))
      
  (define-syntax parameterize
    ;; Derived from Ikarus's implementation of parameterize.
    (lambda (stx)
      (syntax-case stx ()
        [(_ () b0 b ...)
         #'(let () b0 b ...)]
        [(_ ([p e] ...) b0 b ...)
         (with-syntax ([(tp ...) (generate-temporaries #'(p ...))]
                       [(te ...) (generate-temporaries #'(e ...))])
           #'(let ([tp p] ...
                   [te e] ...)
               (let ([swap (lambda ()
                             (let ([t (tp)])
                               (tp te)
                               (set! te t))
                             ...)])
                 (dynamic-wind
                  swap
                  (lambda () b0 b ...)
                  swap))))])))  

)

