module/ice-9/poe.scm

   1 ;;; installed-scm-file
   2
   3 ;;;;    Copyright (C) 1996, 2001, 2006, 2011 Free Software Foundation, Inc.
   4 ;;;;
   5 ;;;; This library is free software; you can redistribute it and/or
   6 ;;;; modify it under the terms of the GNU Lesser General Public
   7 ;;;; License as published by the Free Software Foundation; either
   8 ;;;; version 3 of the License, or (at your option) any later version.
   9 ;;;;
  10 ;;;; This library is distributed in the hope that it will be useful,
  11 ;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 ;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13 ;;;; Lesser General Public License for more details.
  14 ;;;;
  15 ;;;; You should have received a copy of the GNU Lesser General Public
  16 ;;;; License along with this library; if not, write to the Free Software
  17 ;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  18 ;;;;
  19
  20 \f
  21 (define-module  (ice-9 poe)
  22   :use-module (ice-9 hcons)
  23   :export (pure-funcq perfect-funcq))
  24
  25 \f
  26
  27
  28 ;;; {Pure Functions}
  29 ;;;
  30 ;;; A pure function (of some sort) is characterized by two equality
  31 ;;; relations: one on argument lists and one on return values.
  32 ;;; A pure function is one that when applied to equal arguments lists
  33 ;;; yields equal results.
  34 ;;;
  35 ;;; If the equality relationship on return values can be eq?, it may make
  36 ;;; sense to cache values returned by the function.  Choosing the right
  37 ;;; equality relation on arguments is tricky.
  38 ;;;
  39
  40 \f
  41 ;;; {pure-funcq}
  42 ;;;
  43 ;;; The simplest case of pure functions are those in which results
  44 ;;; are only certainly eq? if all of the arguments are.  These functions
  45 ;;; are called "pure-funcq", for obvious reasons.
  46 ;;;
  47
  48
  49 (define funcq-memo (make-weak-key-hash-table 523)) ; !!! randomly selected values
  50 (define funcq-buffer (make-gc-buffer 256))
  51
  52 (define (funcq-hash arg-list n)
  53   (let ((it (let loop ((x 0)
  54                        (arg-list arg-list))
  55               (if (null? arg-list)
  56                   (modulo x n)
  57                   (loop (logior x (hashq (car arg-list) 4194303))
  58                         (cdr arg-list))))))
  59     it))
  60
  61 ;; return true if lists X and Y are the same length and each element is `eq?'
  62 (define (eq?-list x y)
  63   (if (null? x)
  64       (null? y)
  65       (and (not (null? y))
  66            (eq? (car x) (car y))
  67            (eq?-list (cdr x) (cdr y)))))
  68
  69 (define (funcq-assoc arg-list alist)
  70   (if (null? alist)
  71       #f
  72       (if (eq?-list arg-list (caar alist))
  73           (car alist)
  74           (funcq-assoc arg-list (cdr alist)))))
  75
  76
  77 (define not-found (list 'not-found))
  78
  79
  80 (define (pure-funcq base-func)
  81   (lambda args
  82     (let* ((key (cons base-func args))
  83            (cached (hashx-ref funcq-hash funcq-assoc funcq-memo key not-found)))
  84       (if (not (eq? cached not-found))
  85           (begin
  86             (funcq-buffer key)
  87             cached)
  88
  89           (let ((val (apply base-func args)))
  90             (funcq-buffer key)
  91             (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
  92             val)))))
  93
  94 \f
  95
  96 ;;; {Perfect funq}
  97 ;;;
  98 ;;; A pure funq may sometimes forget its past but a perfect
  99 ;;; funcq never does.
 100 ;;;
 101
 102 (define (perfect-funcq size base-func)
 103   (define funcq-memo (make-hash-table size))
 104
 105   (lambda args
 106     (let* ((key (cons base-func args))
 107            (cached (hashx-ref funcq-hash funcq-assoc funcq-memo key not-found)))
 108       (if (not (eq? cached not-found))
 109           (begin
 110             (funcq-buffer key)
 111             cached)
 112
 113           (let ((val (apply base-func args)))
 114             (funcq-buffer key)
 115             (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
 116             val)))))