Match multiple union types

[janwirth]

twop/ts-union provides the experimental matchWith syntax: https://github.com/twop/ts-union#experimental-matchwith

const State = Union({
  Loading: of(null),
  Loaded: of<number>(),
  Err: of<string>(),
});

const Ev = Union({
  ErrorHappened: of<string>(),
  DataFetched: of<number>(),
});

const { Loaded, Err, Loading } = State;

const transition = (prev: typeof State.T, ev: typeof Ev.T) =>
  State.match(prev, {
    Loading: () =>
      Ev.match(ev, {
        ErrorHappened: (err) => Err(err),
        DataFetched: (data) => Loaded(data),
      }),

    Loaded: (loadedData) =>
      // just add to the current loaded value as an example
      Ev.if.DataFetched(
        ev,
        (data) => Loaded(loadedData + data),
        () => prev
      ),

    default: (s) => s,
  });

Are ther any plans to support this? Maybe the following syntax could work:

matchExhaustive(dataAorB, {
   a: matchWith(dataCorD, {
      // matchWith puts the args to the constructors in an array
      c: ([[aPayload, aPayload2nd], [cPayload]]) => cPayload && aPaylod2,
      d: () => false
   }
   b: (arg) => true // or wildcard
})

This is probably very tricky to type, isn't it?

[felixschorer]

The focus of this module is to reduce the boilerplate code when defining discriminating unions, i.e. type guards and constructor functions. I've included some form of pattern matching as a convenience. If you need more advanced pattern matching, I suggest you give ts-pattern a try.

import { impl, tag, Variant } from "@practical-fp/union-types"
import { __, match, select } from 'ts-pattern'

type State =
  | Variant<"Loading">
  | Variant<"Loaded", number>
  | Variant<"Err", string>

const { Loading, Loaded, Err } = impl<State>()

type Ev =
  | Variant<"ErrorHappened", string>
  | Variant<"DataFetched", number>

const { ErrorHappened, DataFetched } = impl<Ev>()

function transition(prev: State, ev: Ev) {
  return match([prev, ev] as const)
    .with([
        tag("Loading"), 
        tag("ErrorHappened", select()),
    ], err => Err(err))
    .with([
        tag("Loading"), 
        tag("DataFetched", select()),
    ], data => Loaded(data))
    .with([
        tag("Loaded", select("loadedData")), 
        tag("DataFetched", select("data")),
    ], ({ loadedData, data }) => Loaded(loadedData + data))
    .with(__, () => prev)
    .exhaustive()
}

If you don't want to repeat the tags all the time, that would also be possible.

function transition(prev: State, ev: Ev) {
  return match([prev, ev] as const)
    .with([
        Loading(), 
        tag(ErrorHappened.tag, select()),
    ], err => Err(err))
    .with([
        Loading(), 
        tag(DataFetched.tag, select()),
    ], data => Loaded(data))
    .with([
        tag(Loaded.tag, select("loadedData")), 
        tag(DataFetched.tag, select("data")),
    ], ({ loadedData, data }) => Loaded(loadedData + data))
    .with(__, () => prev)
    .exhaustive()
}

I guess one could also extend impl<>() to enable this syntax. Perhaps I could do that in version 2.

function transition(prev: State, ev: Ev) {
  return match([prev, ev] as const)
    .with([
        Loading(),
        ErrorHappened.select(),
    ], err => Err(err))
    .with([
        Loading(), 
        DataFetched.select(),
    ], data => Loaded(data))
    .with([
       Loaded.select("loadedData"), 
       DataFetched.select("data"),
    ], ({ loadedData, data }) => Loaded(loadedData + data))
    .with(__, () => prev)
    .exhaustive()
}

[felixschorer]

I've created the v2 branch and published version 2.0.0-alpha1 to evaluate if an integration with ts-pattern makes any sense.

[anilanar]

Hmm, what version of ts-pattern do you rely on?

import { AnonymousSelectPattern, NamedSelectPattern } from "ts-pattern/lib/types/Pattern", I don't see a NamedSelectPattern type in ts-pattern v3.

[felixschorer]

@anilanar 3.3.3 according to the lock file.

[felixschorer]

I am currently working on a rewrite of the library. It will support discriminating union types of all shapes and forms (excluding tuples). As a side effect of this, I had to come up with a new matching syntax. The new syntax will allow matching on tuples.

type Union = Variant<"Foo", string> | Variant<"Bar", number>

const { Foo, Bar } = impl<Union>()

const tuple: [Union, Union] = [Bar(42), Foo("Hello, World!")]

const t = match2(tuple)
    .with([Foo, Foo], ([foo, foo]) => /* ... */)
    .with([Foo, Bar], ([foo, bar]) => /* ... */)
    .with([Bar, null], ([bar, rest]) => /* ... */)
    .done()

[felixschorer]

I've published version 2.0.0-alpha2 to evaluate the improved matching. It doesn't rely on ts-pattern anymore.

It also adds implFactory to create implementations for discriminating union types of different shapes.