Editing Immutable object (section)

=== Racket ===
[[Racket (programming language)|Racket]] substantially diverges from other [[Scheme (programming language)|Scheme]] implementations by making its core pair type ("cons cells") immutable. Instead, it provides a parallel mutable pair type, via <code>mcons</code>, <code>mcar</code>, <code>set-mcar!</code> etc. In addition, many immutable types are supported, for example, immutable strings and vectors, and these are used extensively. New structs are immutable by default, unless a field is specifically declared mutable, or the whole struct:
<syntaxhighlight lang="racket">
(struct foo1 (x y))             ; all fields immutable
(struct foo2 (x [y #:mutable])) ; one mutable field
(struct foo3 (x y) #:mutable)   ; all fields mutable
</syntaxhighlight>
The language also supports immutable hash tables, implemented functionally, and immutable dictionaries.