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(define-library (csc list)
(export
enumerate
filter
intercalate
revappend
split-at
take
unzip)
(import (scheme base)
(only (csc loop)
loop
return)
(only (csc match)
match))
(begin
(define (take n xs)
(loop for x in xs
for i from 1 to n
collect x))
(define (split-at n xs)
(if (<= n 0)
(values '() xs)
(loop for i from 1 to n
for x in xs
for second-half = (cdr xs) then (cdr second-half)
collect x into first-half
finally (return (values first-half second-half)))))
(define (revappend a b)
(let loop ((xs a)
(acc b))
(if (null? xs)
acc
(loop (cdr xs) (cons (car xs) acc)))))
(define (intercalate x l)
(match l
('() '())
((_) l)
((head . tail) (cons head (cons x (intercalate x tail))))))
(define (enumerate l)
(let loop ((i 0)
(l l))
(match l
('() '())
((head . tail) (cons (cons i head) (loop (+ 1 i) tail))))))
(define (filter p l)
(let loop ((l l)
(acc '()))
(match l
('() (reverse acc))
((x . xs)
(if (p x)
(loop xs (cons x acc))
(loop xs acc))))))
(define (unzip l)
(loop for x in l
collect (car x) into xs
collect (cdr x) into ys
finally (return (values xs ys))))))
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