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

Question

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?

Answer 1

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.

Answer 2

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.