How to parse a function into another function in scheme

Question

I am trying to create a function that takes another function as a parameter and calls the functions in a loop.

The code below should get the function and the number of times the loop should execute:

(define (forLoop loopBody reps)
    (let
        (
         (fun (car loopBody))
         (str (cdr loopBody))
        )
       (cond
          ((eval (= reps 0) (interaction-environment)) "")
          (else (cons str (forLoop '(fun str) (- reps 1))))
       )
    )
)

The code below is how i am calling the function

(define (printermsg)
  (display msg)
 )
(forLoop '(printer "text ") 4)

The expected output for the above code:

text text text text

You should not format and indent your code opening and closing parentheses `()` in a line by themselves, they're not brackets `{}` you know :) . That's not the usual style to do it, a good IDE will help you to balance your parentheses. See my answer for the recommended way to format Scheme code. — Óscar López, Oct 22 '21 at 16:56

score 0 · Accepted Answer · answered Oct 22 '21 at 15:11

There are several issues with your code:

The way you pass the loopBody parameter to your function is incorrect, a list of symbols will not work: you want to pass along a list with the actual function and its argument.
You are not obtaining the second element in the list, cdr won't work because it returns the rest of the list, you need to use cadr instead.
Avoid using eval, it's not necessary at all for what you want, and in general is a bad idea.
Why are you building a list? cons is not required here.
When calling the recursion, you are again passing a list of symbols instead of the proper argument.
You're not actually calling the function!

This should work:

(define (forLoop loopBody reps)
  (let ((fun (car  loopBody))
        (str (cadr loopBody)))
    (cond
      ((= reps 0) (void)) ; don't do anything when the loop is done
      (else
       (fun str) ; actually call the function!
       (forLoop loopBody (- reps 1))))))

(define (printer msg)
  (display msg))

(forLoop (list printer "text ") 4) ; see how to build the loop body
=> text text text text

How to parse a function into another function in scheme

1 Answers1