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(functor ((algebraic-structures foldable) (F (fold)))
(fold
length
count
any
every)
(import (except scheme length)
F
(only (chicken base) add1 call/cc))
(define (length xs)
(fold (lambda (_ acc) (add1 acc))
0
xs))
(define (count p? xs)
(fold (lambda (e acc)
(if (p? e)
(add1 acc)
acc))
0
xs))
(define (any pred xs)
(call/cc
(lambda (return)
(fold (lambda (e acc)
(cond ((pred e) => return)
(else acc)))
#f
xs))))
(define (every pred xs)
(call/cc
(lambda (return)
(fold (lambda (e acc)
(or (pred e) (return #f)))
#t
xs)))))
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