Extensible Variant
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.
Definition and Usage
The ..
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 +=
operator!
Pattern Matching Caveats
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 switch
expression.
Tips & Tricks
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.