withIn/withOut methods for constraining one side of a morph

[ssalbdivad]

This would allow the inputs or outputs of morphs to be directly constrained via chaining as opposed to having to intersect (for input) or pipe (for output) from scratch.

I did some initial work defining the API, but there are nuances around how branches are mapped when the input is a union that would be tough to capture so will require a bit more thought:

// in ark/type/methods/morph.ts

interface Type<out t = unknown, $ = {}> extends BaseTyp<t, $> {
  // other methods

  withIn<mappedIn extends this["inferBrandableIn"]>(
    mapIn: (In: this["in"]) => BaseType & { inferBrandableIn: mappedIn }
  ): Type<
    this["inferIntrospectableOut"] extends this["inferBrandableOut"]
      ? (In: mappedIn) => To<this["inferBrandableOut"]>
      : (In: mappedIn) => Out<this["inferBrandableOut"]>,
    $
  >;

  withOut<mappedOut extends this["inferBrandableOut"]>(
    mapOut: (In: this["out"]) => BaseType & { inferBrandableOut: mappedOut }
  ): Type<(In: this["inferBrandableIn"]) => To<mappedOut>, $>;
}

I also wrote an initial test:

it("withIn", () => {
  const t = type("string")
    .pipe((s) => s.length)
    .withIn((t) => t.atLeastLength(1));

  const expected = type("string >= 1").pipe((s) => s.length);

  attest(t.expression).equals(expected.expression);
  attest<typeof expected.t>(t.t);
});