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; -*- Mode: Scheme; Syntax: Scheme; Package: Scheme; -*-
; Copyright (c) 1993-2001 by Richard Kelsey and Jonathan Rees. See file COPYING.
; This is file util.scm.
;;;; Utilities
(define (unspecific) (if #f #f))
(define (reduce cons nil list) ;used by length, append, etc.
(if (null? list)
nil
(cons (car list) (reduce cons nil (cdr list)))))
(define (filter pred lst)
(reduce (lambda (x rest)
(if (pred x) (cons x rest) rest))
'()
lst))
; Position of an object within a list
(define (pos pred)
(lambda (thing l)
(let loop ((i 0) (l l))
(cond ((null? l) #f)
((pred thing (car l)) i)
(else (loop (+ i 1) (cdr l)))))))
(define posq (pos eq?))
(define posv (pos eqv?))
(define position (pos equal?))
; Is pred true of any element of l?
(define (any pred l)
;; (reduce or #f l), sort of
(if (null? l)
#f
(or (pred (car l))
(any pred (cdr l)))))
; Is pred true of every element of l?
(define (every pred l)
;; (reduce and #t l), sort of
(if (null? l)
#t
(and (pred (car l)) (every pred (cdr l)))))
(define (sublist l start end)
(if (> start 0)
(sublist (cdr l) (- start 1) (- end 1))
(let recur ((l l) (end end))
(if (= end 0)
'()
(cons (car l) (recur (cdr l) (- end 1)))))))
(define (last x)
(if (null? (cdr x))
(car x)
(last (cdr x))))
(define (insert x l <)
(cond ((null? l) (list x))
((< x (car l)) (cons x l))
(else (cons (car l) (insert x (cdr l) <)))))
;----------------
; Variations on a theme.
;
; FOLD is a tail-recursive version of REDUCE.
(define (fold folder list accumulator)
(do ((list list (cdr list))
(accum accumulator (folder (car list) accum)))
((null? list)
accum)))
(define (fold->2 folder list acc0 acc1)
(let loop ((list list) (acc0 acc0) (acc1 acc1))
(if (null? list)
(values acc0 acc1)
(call-with-values
(lambda ()
(folder (car list) acc0 acc1))
(lambda (acc0 acc1)
(loop (cdr list) acc0 acc1))))))
(define (fold->3 folder list acc0 acc1 acc2)
(let loop ((list list) (acc0 acc0) (acc1 acc1) (acc2 acc2))
(if (null? list)
(values acc0 acc1 acc2)
(call-with-values
(lambda ()
(folder (car list) acc0 acc1 acc2))
(lambda (acc0 acc1 acc2)
(loop (cdr list) acc0 acc1 acc2))))))
;----------------
; A version of LET and LET* which allows clauses that return multiple values.
;
; There is another copy of this in big/mvlet.scm.
;
; MV = multiple-value
;
; (mvlet (<clause> ...) <body>)
; (mvlet* (<clause> ...) <body>)
;
; <clause> ::= (<ids> <expression>)
; <ids> ::= <id> | (<id> ...) | (<id> ... . <id>)
;
; A clause of the form (<id> <exp>) is like a normal LET clause. There is no
; clause equivalent to
; (call-with-values (lambda () <expression>)
; (lambda <id> <body>))
(define-syntax mvlet
(syntax-rules ()
((mvlet () body ...)
(let () body ...))
((mvlet (clause ...) body ...)
(mvlet-helper (clause ...) () (body ...)))))
(define-syntax mvlet-helper
(syntax-rules ()
((mvlet-helper () clauses (body ...))
(let clauses body ...))
((mvlet-helper (((var . more-vars) val) more ...) clauses body)
(copy-vars (var . more-vars) () val (more ...) clauses body))
((mvlet-helper ((var val) more ...) clauses body)
(mvlet-helper (more ...) ((var val) . clauses) body))))
(define-syntax copy-vars
(syntax-rules ()
((copy-vars (var . more-vars) (copies ...)
val more clauses body)
(copy-vars more-vars (copies ... x)
val more ((var x) . clauses) body))
((copy-vars () copies val more clauses body)
(call-with-values
(lambda () val)
(lambda copies
(mvlet-helper more clauses body))))
((copy-vars last (copies ...) val more clauses body)
(call-with-values
(lambda () val)
(lambda (copies ... . lastx)
(mvlet-helper more ((last lastx) . clauses) body))))))
(define-syntax mvlet*
(syntax-rules ()
((mvlet* () body ...)
(let () body ...))
((mvlet* (((vars ...) val) clause ...) body ...)
(call-with-values
(lambda () val)
(lambda (vars ...)
(mvlet (clause ...) body ...))))
((mvlet* ((var val) clause ...) body ...)
(let ((var val)) (mvlet (clause ...) body ...)))))
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