Make my own while loop using "define-syntax-rule"

Question

I am trying to create my own while-loop in racket using the "define-syntax-rule". I want it to be procedural based, so no helper functions (i.e. just using lambda, let, letrec, etc.).

I have this, but it gives me some sort of lambda identifier error.

(define-syntax-rule (while condition body)
  (lambda (iterate)
    (lambda (condition body) ( (if condition)
                                   body
                                   iterate))))

I want it to be so I can use it like a regular while loop For Example:

(while (x < 10) (+ x 1))

Calling this will(should) return 10 after the loop is done.

How can my code be fixed to do this?

Your desired syntax doesn’t really make any sense, for a few reasons. First of all, (x < 10) doesn’t make sense, since Scheme uses prefix function application, so it would need to be (< x 10). Second of all, where is x bound? Your current code doesn’t make that clear. Third of all, (+ x 1) doesn’t mutate x, it just produces a new number, just like x + 1 would do in a C-like language. You would need to do (set! x (+ x 1)) to emulate x += 1. Without those clarifications, it’s hard to answer your question at all, even with your attempted code sample. — Alexis King
Maybe you should have a look at an implementation of SRFI-42 "Eager Comprehensions". — Michael Vehrs

user448810 user448810 · Accepted Answer · 2017-02-27T14:07:20

Here is the while from my Standard Prelude, along with an example of its use:

Petite Chez Scheme Version 8.4
Copyright (c) 1985-2011 Cadence Research Systems

> (define-syntax while
    (syntax-rules ()
      ((while pred? body ...)
        (do () ((not pred?)) body ...))))
> (let ((x 4))
    (while (< x 10)
      (set! x (+ x 1)))
    x)
10

You should probably talk to your instructor about your misconceptions involving Scheme.

Make my own while loop using "define-syntax-rule"

1 Answers