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[bpt/guile.git] / srfi / srfi-11.scm

;;; srfi-11.scm --- let-values and let*-values

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;;; Commentary:

;; This module exports two syntax forms: let-values and let*-values.
;;
;; Sample usage:
;;
;;   (let-values (((x y . z) (foo a b))
;;                ((p q) (bar c)))
;;     (baz x y z p q))
;;
;; This binds `x' and `y' to the first to values returned by `foo',
;; `z' to the rest of the values from `foo', and `p' and `q' to the
;; values returned by `bar'.  All of these are available to `baz'.
;;
;; let*-values : let-values :: let* : let
;;
;; This module is fully documented in the Guile Reference Manual.

;;; Code:

(define-module (srfi srfi-11)
  :use-module (ice-9 syncase)
  :export-syntax (let-values let*-values))

(cond-expand-provide (current-module) '(srfi-11))

;;;;;;;;;;;;;;
;; let-values
;;
;; Current approach is to translate
;;
;;   (let-values (((x y . z) (foo a b))
;;                ((p q) (bar c)))
;;     (baz x y z p q))
;;
;; into
;;
;;   (call-with-values (lambda () (foo a b))
;;     (lambda (<tmp-x> <tmp-y> . <tmp-z>)
;;       (call-with-values (lambda () (bar c))
;;         (lambda (<tmp-p> <tmp-q>)
;;           (let ((x <tmp-x>)
;;                 (y <tmp-y>)
;;                 (z <tmp-z>)
;;                 (p <tmp-p>)
;;                 (q <tmp-q>))
;;             (baz x y z p q))))))

;; I originally wrote this as a define-macro, but then I found out
;; that guile's gensym/gentemp was broken, so I tried rewriting it as
;; a syntax-rules statement.
;;
;; Since syntax-rules didn't seem powerful enough to implement
;; let-values in one definition without exposing illegal syntax (or
;; perhaps my brain's just not powerful enough :>).  I tried writing
;; it using a private helper, but that didn't work because the
;; let-values expands outside the scope of this module.  I wonder why
;; syntax-rules wasn't designed to allow "private" patterns or
;; similar...
;;
;; So in the end, I dumped the syntax-rules implementation, reproduced
;; here for posterity, and went with the define-macro one below --
;; gensym/gentemp's got to be fixed anyhow...
;
; (define-syntax let-values-helper
;   (syntax-rules ()
;     ;; Take the vars from one let binding (i.e. the (x y z) from ((x y
;     ;; z) (values 1 2 3)) and turn it in to the corresponding (lambda
;     ;; (<tmp-x> <tmp-y> <tmp-z>) ...) from above, keeping track of the
;     ;; temps you create so you can use them later...
;     ;;
;     ;; I really don't fully understand why the (var-1 var-1) trick
;     ;; works below, but basically, when all those (x x) bindings show
;     ;; up in the final "let", syntax-rules forces a renaming.

;     ((_ "consumer" () lambda-tmps final-let-bindings lv-bindings
;         body ...)
;      (lambda lambda-tmps
;        (let-values-helper "cwv" lv-bindings final-let-bindings body ...)))

;     ((_ "consumer" (var-1 var-2 ...) (lambda-tmp ...) final-let-bindings lv-bindings
;         body ...)
;      (let-values-helper "consumer"
;                         (var-2 ...)
;                         (lambda-tmp ... var-1)
;                         ((var-1 var-1) . final-let-bindings)
;                         lv-bindings
;                         body ...))

;     ((_ "cwv" () final-let-bindings body ...)
;      (let final-let-bindings
;          body ...))

;     ((_ "cwv" ((vars-1 binding-1) other-bindings ...) final-let-bindings
;         body ...)
;      (call-with-values (lambda () binding-1)
;        (let-values-helper "consumer"
;                           vars-1
;                           ()
;                           final-let-bindings
;                           (other-bindings ...)
;                           body ...)))))
;
; (define-syntax let-values
;   (syntax-rules ()
;     ((let-values () body ...)
;      (begin body ...))
;     ((let-values (binding ...) body ...)
;      (let-values-helper "cwv" (binding ...) () body ...))))
;
;
; (define-syntax let-values
;   (letrec-syntax ((build-consumer
;                    ;; Take the vars from one let binding (i.e. the (x
;                    ;; y z) from ((x y z) (values 1 2 3)) and turn it
;                    ;; in to the corresponding (lambda (<tmp-x> <tmp-y>
;                    ;; <tmp-z>) ...) from above.
;                    (syntax-rules ()
;                      ((_ () new-tmps tmp-vars () body ...)
;                       (lambda new-tmps
;                         body ...))
;                      ((_ () new-tmps tmp-vars vars body ...)
;                       (lambda new-tmps
;                         (lv-builder vars tmp-vars body ...)))
;                      ((_ (var-1 var-2 ...) new-tmps tmp-vars vars body ...)
;                       (build-consumer (var-2 ...)
;                                       (tmp-1 . new-tmps)
;                                       ((var-1 tmp-1) . tmp-vars)
;                                       bindings
;                                       body ...))))
;                   (lv-builder
;                    (syntax-rules ()
;                      ((_ () tmp-vars body ...)
;                       (let tmp-vars
;                           body ...))
;                      ((_ ((vars-1 binding-1) (vars-2 binding-2) ...)
;                          tmp-vars
;                          body ...)
;                       (call-with-values (lambda () binding-1)
;                         (build-consumer vars-1
;                                         ()
;                                         tmp-vars
;                                         ((vars-2 binding-2) ...)
;                                         body ...))))))
;
;     (syntax-rules ()
;       ((_ () body ...)
;        (begin body ...))
;       ((_ ((vars binding) ...) body ...)
;        (lv-builder ((vars binding) ...) () body ...)))))

;; FIXME: This is currently somewhat unsafe (b/c gentemp/gensym is
;; broken -- right now (as of 1.4.1, it doesn't generate unique
;; symbols)
(define-macro (let-values vars . body)

  (define (map-1-dot proc elts)
    ;; map over one optionally dotted (a b c . d) list, producing an
    ;; optionally dotted result.
    (cond
     ((null? elts) '())
     ((pair? elts) (cons (proc (car elts)) (map-1-dot proc (cdr elts))))
     (else (proc elts))))

  (define (undot-list lst)
    ;; produce a non-dotted list from a possibly dotted list.
    (cond
     ((null? lst) '())
     ((pair? lst) (cons (car lst) (undot-list (cdr lst))))
     (else (list lst))))

  (define (let-values-helper vars body prev-let-vars)
    (let* ((var-binding (car vars))
           (new-tmps (map-1-dot (lambda (sym) (gensym))
                                (car var-binding)))
           (let-vars (map (lambda (sym tmp) (list sym tmp))
                          (undot-list (car var-binding))
                          (undot-list new-tmps))))

      (if (null? (cdr vars))
          `(call-with-values (lambda () ,(cadr var-binding))
             (lambda ,new-tmps
               (let ,(apply append let-vars prev-let-vars)
                 ,@body)))
          `(call-with-values (lambda () ,(cadr var-binding))
             (lambda ,new-tmps
               ,(let-values-helper (cdr vars) body
                                   (cons let-vars prev-let-vars)))))))

  (if (null? vars)
      `(begin ,@body)
      (let-values-helper vars body '())))

;;;;;;;;;;;;;;
;; let*-values
;;
;; Current approach is to translate
;;
;;   (let*-values (((x y z) (foo a b))
;;                ((p q) (bar c)))
;;     (baz x y z p q))
;;
;; into
;;
;;   (call-with-values (lambda () (foo a b))
;;     (lambda (x y z)
;;       (call-with-values (lambda (bar c))
;;         (lambda (p q)
;;           (baz x y z p q)))))

(define-syntax let*-values
  (syntax-rules ()
    ((let*-values () body ...)
     (begin body ...))
    ((let*-values ((vars-1 binding-1) (vars-2 binding-2) ...) body ...)
     (call-with-values (lambda () binding-1)
       (lambda vars-1
         (let*-values ((vars-2 binding-2) ...)
           body ...))))))

; Alternate define-macro implementation...
;
; (define-macro (let*-values vars . body)
;   (define (let-values-helper vars body)
;     (let ((var-binding (car vars)))
;       (if (null? (cdr vars))
;           `(call-with-values (lambda () ,(cadr var-binding))
;              (lambda ,(car var-binding)
;                ,@body))
;           `(call-with-values (lambda () ,(cadr var-binding))
;              (lambda ,(car var-binding)
;                ,(let-values-helper (cdr vars) body))))))

;   (if (null? vars)
;       `(begin ,@body)
;       (let-values-helper vars body)))

;;; srfi-11.scm ends here