Variant types are usually constrained to a fixed set of constructors. There may be very rare cases where you still want to be able to add constructors to a variant type even after its initial type declaration. For this, we offer extensible variant types.
.. in the type declaration above defines an extensible variant
type t. The
+= operator is then used to add constructors to the given type.
Note: Don't forget the leading
type keyword when using the
Extensible variants are open-ended, so the compiler will not be able to exhaustively pattern match all available cases. You will always need to provide a default
_ case for every
Fun fact: In ReScript, exceptions are actually extensible variants under the hood, so
exception UserError(string) is equivalent to
type exn += UserError(string). It's one of the very few use-case where extensible variants make sense.
We usually recommend sticking with common variants as much as possible to reap the benefits of exhaustive pattern matching.