| title | description | canonical |
|---|---|---|
Extensible Variant | Extensible Variants in ReScript | /docs/manual/v11.0.0/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.
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typet=..typet+=Othertypet+= | Point(float, float) | Line(float, float, float, float)varCaml_exceptions=require("./stdlib/caml_exceptions.js");varOther=Caml_exceptions.create("Playground.Other");varPoint=Caml_exceptions.create("Playground.Point");varLine=Caml_exceptions.create("Playground.Line");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!
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.
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letprint=v=>switchv { | Point(x, y) =>Console.log2("Point", (x, y)) | Line(ax, ay, bx, by) =>Console.log2("Line", (ax, ay, bx, by)) | Other | _=>Console.log("Other") }functionprint(v){if(v.RE_EXN_ID===Point){console.log("Point",[v._1,v._2]);}elseif(v.RE_EXN_ID===Line){console.log("Line",[v._1,v._2,v._3,v._4]);}else{console.log("Other");}}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.
