unbound identifier in module - DrRacket

Question

I am learning scheme and working with the metacircular interpreter for examples and practice, and I am getting the following error:

 set-car!: unbound identifier in module in: set-car!

And the error is being caused by the following code:

(define (add-binding-to-frame! var val frame)
  (set-car! frame (cons var (car frame)))
  (set-cdr! frame (cons val (cdr frame))))

I have two questions... First, what does the error mean? And secondly, am I missing a let function?

Check this question, it might help http://stackoverflow.com/q/9475366/1197333 — Ivancho, Nov 21 '14 at 18:15
#!racket (a completely different language than Scheme) does not have `set-car!` and `set-cdr!`. Use #!r6rs instead. — Sylwester, Nov 21 '14 at 18:18

soegaard · Accepted Answer · 2014-11-21T18:56:44.193

2

In Racket a pair allocated by cons is immutable by design - therefore there are no set-car!. To allocated a mutable pair, use mcons. To mutate a mutable pair, use set-mcar! and set-mcdr!.

Here is a complete program:

#lang racket
(define p (mcons 1 2))
(set-mcar! p 3)
p

edited Nov 21 '14 at 18:56

answered Nov 21 '14 at 18:49

soegaard

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Made the appropriate changes, and I still get the same error. Thank you for the answer though. – choloboy Nov 21 '14 at 18:52
1

I have added a complete program. Note that mcons, mcar, mcdr, set-mcar! and set-mcdr are exported by racket/base. Use (require (only-in racket/base mcons mcar mcdr set-mcar! set-mcdr!)) if you are using a language, where the mutable versions aren't included from the start. – soegaard Nov 21 '14 at 18:58

unbound identifier in module - DrRacket

1 Answers1