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(define-library (csc list)
(export
enumerate
filter
intercalate
revappend
split-at
take)
(import (scheme base)
(only (csc match)
match))
(begin
(define (take n xs)
(let loop ((n n)
(xs xs)
(acc '()))
(if (or (not (positive? n)) (null? xs))
(reverse acc)
(loop (- n 1) (cdr xs) (cons (car xs) acc)))))
(define (split-at n xs)
(let loop ((n n)
(xs xs)
(acc '()))
(if (or (not (positive? n)) (null? xs))
(values (reverse acc) xs)
(loop (- n 1) (cdr xs) (cons (car xs) acc)))))
(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))))))))
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