scheme48/scheme/srfi/srfi-5.scm

; Part of Scheme 48 1.9.  See file COPYING for notices and license.

; Authors: Richard Kelsey, Jonathan Rees, Mike Sperber

; Rewritten, simplified, and corrected from the SRFI document.
;
; The SRFI implementation gets the scoping wrong for the name.  It is visible
; to the arguments and should not be.

(define-syntax let
  (syntax-rules ()

    ; If no name we go straight to the standard LET.
    ((let () body ...)
     (standard-let () body ...))
    ((let ((variable value) ...) body ...)
     (standard-let ((variable value) ...) body ...))
    ; Rest binding
    ((let ((var val) . bindings) body ...)
     (let-loop #f bindings (var) (val) (body ...)))

    ;; Signature-style and standard named LET.
    ((let (name bindings ...) body ...)
     (let-loop name (bindings ...) () () (body ...)))
    ((let name bindings body ...)
     (let-loop name bindings () () (body ...)))))

; A loop to walk down the list of bindings.

(define-syntax let-loop
  (syntax-rules ()

    ; No more bindings - make a LETREC.
    ((let-loop name () (vars ...) (vals ...) body)
     ((letrec ((name (lambda (vars ...) . body)))
        name)
      vals ...))

    ; Rest binding, no name
    ((let-loop #f (rest-var rest-val ...) (var ...) (val ...) body)
     (standard-let ((var val) ... (rest-var (list rest-val ...))) . body))

    ; Process a (var val) pair.
    ((let-loop name ((var val) more ...) (vars ...)     (vals ...)     body)
     (let-loop name (more ...)           (vars ... var) (vals ... val) body))
    
    ; End with a rest variable - make a LETREC.
    ((let-loop name (rest-var rest-vals ...) (vars ...) (vals ...) body)
     ((letrec ((name (lambda (vars ... . rest-var) . body)))
       name)
      vals ... rest-vals ...))))

; Four loops - normal and `signature-style', each with and without a rest
; binding.
;
;(let fibonacci ((n 10) (i 0) (f0 0) (f1 1))
;  (if (= i n)
;      f0
;      (fibonacci n (+ i 1) f1 (+ f0 f1))))
;
;(let (fibonacci (n 10) (i 0) (f0 0) (f1 1))
;  (if (= i n)
;      f0
;      (fibonacci n (+ i 1) f1 (+ f0 f1))))
;
;(let fibonacci ((n 10) (i 0) . (f 0 1))
;  (if (= i n)
;      (car f)
;      (fibonacci n (+ i 1) (cadr f) (+ (car f) (cadr f)))))
;
;(let (fibonacci (n 10) (i 0) . (f 0 1))
;  (if (= i n)
;      (car f)
;      (fibonacci n (+ i 1) (cadr f) (+ (car f) (cadr f)))))