Investigate structured conditional computations

[kasperisager]

#394 (comment)

Inspired by https://www.staff.ncl.ac.uk/andrey.mokhov/selective-functors.pdf, I drafted this API:

interface Selective<S, T = never> {
  value: Either<S, T>;

  when<P extends S, U>(
    when: Refinement<S, P>,
    mapper: Mapper<P, U>
  ): Selective<S, T | U>;

  get(): T;

  getOr<U>(value: U): T | U;

  getOrElse<U>(thunk: Thunk<U>): T | U;
};

The idea is that Selective models selective function application of type S -> T. Given a type refinement S -> P, Selective#when() applies a function P -> U, producing a new Selective of type S -> T | U. It looks like this in action:

declare const string: Selective<string>;

declare const isFoo: Refinement<string, "foo">;

declare const isBar: Refinement<string, "bar">;

const number = string
  // If string is "foo", produce 42
  .when(isFoo, (foo) => 42)
  // If string is "bar", produce 56
  .when(isBar, (bar) => 56)
  // If string is neither "bar" nor "foo", produce 23
  .getOr(23);