set-car!, set-cdr! unbound in racket?

20
votes

I am just trying to do very simple code with set-car! and set-cdr! in racket, but I got the error: expand: unbound identifier in module in: set-car! and expand: unbound identifier in module in: set-cdr!

Aren't they defined in racket? Could anyone help?

2
schemeracket
Which language do you choose? Set the language to R5RS. - pjhades
I have #lang racket in the beginning, when I set #lang R5RS, it gives me an error: Module Language: invalid module text standard-module-name-resolver: collection not found: #<path:R5RS/lang> in any of: (#<path:/home/chengtie/.racket/5.1.1/collects> #<path:/usr/share/racket/collects>) - SoftTimur
Use #lang r5rs -- lower case. (But using the r5rs language is not going to make for a pleasant experience.) - Eli Barzilay

2 Answers

31
votes

You need to import mutable-pairs-6, like this:

(require rnrs/mutable-pairs-6)

Those procedures were moved to a different module and renamed to mcons, mcar, mcdr, set-mcar!, set-mcdr!, mlist to emphasize that they operate on mutable data, unlike their immutable counterparts.

12
votes

Óscar López's answer is correct, but doesn't explain why normal pairs are immutable.

In Racket (and its predecessor, PLT Scheme, since version 4), cons cells are immutable by default. Most Scheme programs never need to modify cons cells, and having them be immutable allows many optimisations. (For example, list? and length can both be constant-time.)

For cases where mutability is needed, there's mpair (as mentioned), and more usefully, there's boxes.