The `immutable_arrays` extension

Immutable arrays are just like regular arrays, but they can’t be mutated: there’s no iarr.(0) <- new_value, no Iarray.blit, etc. The syntax is the same as existing mutable arrays, but using : instead of |:

open Iarray.O

let immutable_array : string iarray =
  [: "zero"; "one"; "two"; "three" :]
;;

let zero : string =
  immutable_array.:(0)
;;

Because iarrays do not allow mutation, it is possible for the contents of an iarray to be stack allocated. See the Iarray.Local module in the stdlib library.

Another difference is that iarray is covariant: if the type sub is a subtype of the type super, then sub iarray is a subtype of super iarray, as though you had type +'a iarray = .... (Put another way, this means that you can cast through iarrays: (iarr : sub iarray :> super iarray) is always valid when (x : sub :> super) is.)

You can also have immutable array comprehensions: [: x, y for x = 1 to 3 and y in [: "some"; "thing" :] :].

This extension will appear in OCaml 5.4 as the Iarray module. See ocaml/ocaml#13097.

Topics

The immutable_arrays extension

The `immutable_arrays` extension