ice-9/poe.scm

   1 ;;; installed-scm-file
   2
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  44
  45 \f
  46 (define-module  (ice-9 poe)
  47   :use-module (ice-9 hcons))
  48
  49 \f
  50
  51
  52 ;;; {Pure Functions}
  53 ;;;
  54 ;;; A pure function (of some sort) is characterized by two equality
  55 ;;; relations: one on argument lists and one on return values.
  56 ;;; A pure function is one that when applied to equal arguments lists
  57 ;;; yields equal results.
  58 ;;;
  59 ;;; If the equality relationship on return values can be eq?, it may make
  60 ;;; sense to cache values returned by the function.  Choosing the right
  61 ;;; equality relation on arguments is tricky.
  62 ;;;
  63
  64 \f
  65 ;;; {pure-funcq}
  66 ;;;
  67 ;;; The simplest case of pure functions are those in which results
  68 ;;; are only certainly eq? if all of the arguments are.  These functions
  69 ;;; are called "pure-funcq", for obvious reasons.
  70 ;;;
  71
  72
  73 (define funcq-memo (make-weak-key-hash-table 523)) ; !!! randomly selected values
  74 (define funcq-buffer (make-gc-buffer 256))
  75
  76 (define (funcq-hash arg-list n)
  77   (let ((it (let loop ((x 0)
  78                        (arg-list arg-list))
  79               (if (null? arg-list)
  80                   (modulo x n)
  81                   (loop (logior x (hashq (car arg-list) 4194303))
  82                         (cdr arg-list))))))
  83     it))
  84
  85 (define (funcq-assoc arg-list alist)
  86   (let ((it (and alist
  87                  (let and-map ((key arg-list)
  88                                (entry (caar alist)))
  89                    (or (and (and (not key) (not entry))
  90                             (car alist))
  91                        (and key entry
  92                             (eq? (car key) (car entry))
  93                             (and-map (cdr key) (cdr entry))))))))
  94     it))
  95
  96
  97
  98 (define-public (pure-funcq base-func)
  99   (lambda args
 100     (let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
 101       (if cached
 102           (begin
 103             (funcq-buffer (car cached))
 104             (cdr cached))
 105
 106           (let ((val (apply base-func args))
 107                 (key (cons base-func args)))
 108             (funcq-buffer key)
 109             (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
 110             val)))))
 111
 112 \f
 113
 114 ;;; {Perfect funq}
 115 ;;;
 116 ;;; A pure funq may sometimes forget its past but a perfect
 117 ;;; funcq never does.
 118 ;;;
 119
 120 (define-public (perfect-funcq size base-func)
 121   (define funcq-memo (make-hash-table size))
 122
 123   (lambda args
 124     (let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
 125       (if cached
 126           (begin
 127             (funcq-buffer (car cached))
 128             (cdr cached))
 129
 130           (let ((val (apply base-func args))
 131                 (key (cons base-func args)))
 132             (funcq-buffer key)
 133             (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
 134             val)))))
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