gistfile1.scm

(define foldl (lambda (func accum lst) (if (null? lst) accum (foldl func (func accum (car lst)) (cdr lst)))))

(define append (lambda (list1 list2) (if (null? list1) list2 (cons (car list1) (append (cdr list1) list2)))))

(define filter (lambda (f ls) (foldl  (lambda (e r) (if (f e) (append r (list e)) r)) (quote ()) ls)))


# I'm thinking this will never work, since filter can return an empty list, which will be compared with <.. unless scheme is supposed to be able to compare numbers against empties?

(filter (lambda (e) (< e 0)) (quote (-9 0 2 -2 6 4 -3 -99 201 16 -12)))



Type ,? (comma question-mark) for help.
> (define fldl (lambda (func accum lst) (if (null? lst) accum (foldl func (func accum (car lt)) (cdr lst)))))
; no values returned
> (define append (lambda (list1 list2) (if (null? list1) list2 (cons (car list1) (append (cdr list1) list2)))))
; no values returned
> (define filter (lambda (f ls) (foldl  (lambda (e r) (if f e) (append r (list e)) r)) (quote ()) ls)))
; no values returned
> (filter (lambda (e) (< e 0)) (quote (-9 0 2 -2 6 4 -3 -99 21 16 -12)))

Error: vm-exception
       #f
       (< '() 0)