Scheme filters - “wrong value to apply: #f”

Question

I'm trying to filter out a list based off of a predicate I wrote myself, but when I run the filter, I get

ERROR: Wrong value to apply: #f

The code of the predicate:

;;;Predicate for checking if a string is not empty or full of whitespaces
(define (notwhitespace? str)
  (if (equal? str "") #F (
    (call-with-current-continuation
     (lambda (return)
      (for-each
       (lambda (c)
         (if (not (char-whitespace? c)) #T #F))
        (string->list str))
        #F))
      )
    )
)

this is my implementation of the filter (it is in a let statement):

(updated-strlist(filter notwhitespace? strlist))

any ideas? thanks!

Answer 1

不要写(#f) ，应该是#f 。

Answer 2

So (call-with-current-continuation ...) in your code is wrappen in extra parentheses which means that Scheme should take the result and run it as a procedure the moment it gets it.

Usually in a LISP evaluator apply is the procedure that runs procedures. eg.

(define (test) (display "hello"))
(define (get-proc) test)
((get-proc)) ; ==> undefined, displays "hello"

You code however tries to do this (#f) and since #f is not a procedure apply cannot run it as if it were one.

A comment on the rest there. If you are not using return you really shouldn't use call-with-current-continuation at all and for-each does sonething entirely different than you think. nowhitespace? will always evaluate to #f when you've fixed your problems because the last expression in the body of the continuation lambda is #f (the returned value).

I guess you are looking for something like:

;; needs to import (srfi :1)
(define (notwhitespace? str)
  (every (lambda (x) (not (char-whitespace? x)))
         (list->string str)))

;; needs to import (srfi :13)
(define (notwhitespace2? str)
  (not (string-index str char-whitespace?)))