Derive sum types from codecs

sum-io-ts-codecs.ts

import * as Sum from 'shared/facades/Sum';
import * as t from 'shared/facades/io-ts';

const TypeA = t.strict({
  a: t.string,
});

interface TypeA extends t.TypeOf<typeof TypeA> {}

const TypeB = t.strict({
  b: t.number,
});

interface TypeB extends t.TypeOf<typeof TypeB> {}

// ------------------------
// Version 1
// ------------------------

type X = Sum.Member<'A', TypeA> | Sum.Member<'B', TypeB>;
const X = Sum.create<X>();
const Codec1 = Sum.getCodec(X)({
  A: TypeA,
  B: TypeB,
});

// ------------------------
// Version 2
// ------------------------

type IntersectionToUnion<T> = T extends infer I
  ? { [K in keyof I]: I[K] }[keyof I]
  : never;

const Codec2 = t.strict({
  A: TypeA,
  B: TypeB,
});

type T = t.TypeOf<typeof Codec2>;
type S<A> = {
  [K in keyof A]: Sum.Member<K, A[K]>;
};

type S2 = IntersectionToUnion<S<T>>;
const S2 = Sum.create<S2>();

S2.mk.A({ a: 'a' });