How do you emulate ADTs and pattern matching in TypeScript?
Here's an alternative to the very good answer by @thSoft. On the plus side, this alternative
- has potential interoperability with raw javascript objects on the form
{ type : string } & T
, where the shape ofT
depends on the value oftype
, - has substantially less per-choice boilerplate;
on the negative side
- does not enforce statically that you match all cases,
- does not distinguish between different ADTs.
It looks like this:
// One-time boilerplate, used by all cases.
interface Maybe<T> { value : T }
interface Matcher<T> { (union : Union) : Maybe<T> }
interface Union { type : string }
class Case<T> {
name : string;
constructor(name: string) {
this.name = name;
}
_ = (data: T) => ( <Union>({ type : this.name, data : data }) )
$ =
<U>(f:(t:T) => U) => (union : Union) =>
union.type === this.name
? { value : f((<any>union).data) }
: null
}
function match<T>(union : Union, destructors : Matcher<T> [], t : T = null)
{
for (const destructor of destructors) {
const option = destructor(union);
if (option)
return option.value;
}
return t;
}
function any<T>(f:() => T) : Matcher<T> {
return x => ({ value : f() });
}
// Usage. Define cases.
const A = new Case<number>("A");
const B = new Case<string>("B");
// Construct values.
const a = A._(0);
const b = B._("foo");
// Destruct values.
function f(union : Union) {
match(union, [
A.$(x => console.log(`A : ${x}`))
, B.$(y => console.log(`B : ${y}`))
, any (() => console.log(`default case`))
])
}
f(a);
f(b);
f(<any>{});
Example to illustrate the accepted answer:
enum ActionType { AddItem, RemoveItem, UpdateItem }
type Action =
{type: ActionType.AddItem, content: string} |
{type: ActionType.RemoveItem, index: number} |
{type: ActionType.UpdateItem, index: number, content: string}
function dispatch(action: Action) {
switch(action.type) {
case ActionType.AddItem:
// now TypeScript knows that "action" has only "content" but not "index"
console.log(action.content);
break;
case ActionType.RemoveItem:
// now TypeScript knows that "action" has only "index" but not "content"
console.log(action.index);
break;
default:
}
}
TypeScript 1.4 adds union types and type guards.
I went with the following Visitor-like pattern, inspired by this and this (in the example, a Choice
can be Foo
or Bar
):
interface Choice {
match<T>(cases: ChoiceCases<T>): T;
}
interface ChoiceCases<T> {
foo(foo: Foo): T;
bar(bar: Bar): T;
}
class Foo implements Choice {
match<T>(cases: ChoiceCases<T>): T {
return cases.foo(this);
}
}
class Bar implements Choice {
match<T>(cases: ChoiceCases<T>): T {
return cases.bar(this);
}
}
Usage:
function getName(choice: Choice): string {
return choice.match({
foo: foo => "Foo",
bar: bar => "Bar",
});
}
The matching itself is expressive and type-safe, but there's lot of boilerplate to write for the types.