(define (foldl fn acc ls)
    (if (null? ls)
        (foldl fn (fn acc (car ls)) (cdr ls))))
(define (foldr fn acc ls)
    (if (null? ls)
        (fn (car ls) (foldr fn acc (cdr ls)))))
(define (map fn ls)
    (foldr (lambda (x xs) (cons (fn x) xs)) '() ls))
(define (filter fn ls)
    (foldr (lambda (x xs) (if (fn x) (cons x xs) xs)) '() ls))
(define (double n) (+ n n))
(define (even?  n) (= 0 (modulo n 2)))
(define (zero?  n) (= 0 n))
(define (sub1   n) (- n 1))
(define (not    b) (if b #f #t))

(foldl + 0 '(1 2 3 4 5)) (map double '(1 2 3 4 5)) (filter even? '(1 2 3 4 5 6 7 8 9 10)) (let* ([a 5] [b (+ a 10)]) (+ b 20)) (letrec ([is-even? (lambda (n) (if (zero? n) #t (is-odd? (sub1 n))))] [is-odd? (lambda (n) (if (zero? n) #f (is-even? (sub1 n))))]) (is-odd? 13))

It's a lisp! Or maybe a scheme! And it's written in idris!

How's it work? Write some scheme in the top pane, hit the 'eval' button, and see the results in the bottom pane.

Check it out