Scheme definition error(short)

Question

This is part of an interpreter I am making. I keep getting this error:

define not allowed in an expression context in: (define ret1 (list->string wl))

I am using DrScheme version 371, language Standard (R5RS).

(define (read-command)
  (set! com '( '() ))
  (set! wl (read-as-list))
  (define ret1 (list->string wl))
  (commRead)
  ret1
 )

similar issue here:

    (define repl(
                 lambda()
                  (display "\nUofL>")
                  (define inp (read-command))
                  (define lengtha (length com)

Answer 1

In you interpreter, it seems that definitions can only appear at the beginning of the function. You should use a let* instead:

(define (read-command)
  (let* ((com '('())) ; are you sure you didn't mean '(()) ?
         (wl (read-as-list))
         (ret1 (list->string wl)))
  (commRead ret1)))

For the second problem, try this:

(define repl
  (lambda ()
    (display "\nUofL>")
    (let ((inp (read-command))
          (lengtha (length com)))
      ; return a value here
      )))

As a side note, your code appears to be written in a procedural style - with all those set! and function calls executed for the effect. How on earth is ret1 going to be modified if you don't pass it as a parameter to commRead? I'd suggest you read a good book on Scheme programming and start writing code on a more functional style, currently your code isn't idiomatic and you'll get into trouble sooner or later.