|
| 1 | +--- |
| 2 | +title: "Extensible Variant" |
| 3 | +description: "Extensible Variants in ReScript" |
| 4 | +canonical: "/docs/manual/latest/extensible-variant" |
| 5 | +--- |
| 6 | + |
| 7 | +# Extensible Variant Types |
| 8 | + |
| 9 | +Variants can be made extensible by defining a type using `..` and adding new variant constructors using `+=`. |
| 10 | + |
| 11 | +<CodeTab labels={["ReScript", "JS Output"]}> |
| 12 | + |
| 13 | +```res example |
| 14 | +type t = .. |
| 15 | +
|
| 16 | +type t += Other |
| 17 | +
|
| 18 | +type t += |
| 19 | + | Point(float, float) |
| 20 | + | Line(float, float, float, float) |
| 21 | +``` |
| 22 | +```js |
| 23 | +var Caml_exceptions = require("./stdlib/caml_exceptions.js"); |
| 24 | + |
| 25 | +var Other = Caml_exceptions.create("Playground.Other"); |
| 26 | + |
| 27 | +var Point = Caml_exceptions.create("Playground.Point"); |
| 28 | + |
| 29 | +var Line = Caml_exceptions.create("Playground.Line"); |
| 30 | +``` |
| 31 | + |
| 32 | +</CodeTab> |
| 33 | + |
| 34 | +Pattern-matching is possible the same way as with normal variants but there is one caveat: |
| 35 | +With extensible variants, the possibility to check for exhaustiveness vanishes, you always need a default case `_` here. |
| 36 | + |
| 37 | + |
| 38 | +<CodeTab labels={["ReScript", "JS Output"]}> |
| 39 | + |
| 40 | + |
| 41 | +```res example |
| 42 | +let print = v => |
| 43 | + switch v { |
| 44 | + | Point(x, y) => Js.log2("Point", (x, y)) |
| 45 | + | Line(ax, ay, bx, by) => Js.log2("Line", (ax, ay, bx, by)) |
| 46 | + | Other |
| 47 | + | _ => Js.log("Other") |
| 48 | + } |
| 49 | +``` |
| 50 | +```js |
| 51 | +function print(v) { |
| 52 | + if (v.RE_EXN_ID === Point) { |
| 53 | + console.log("Point", [v._1, v._2]); |
| 54 | + } else if (v.RE_EXN_ID === Line) { |
| 55 | + console.log("Line", [v._1, v._2, v._3, v._4]); |
| 56 | + } else { |
| 57 | + console.log("Other"); |
| 58 | + } |
| 59 | +} |
| 60 | +``` |
| 61 | + |
| 62 | +</CodeTab> |
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