Default values about io-ts HOT 7 CLOSED

export function withDefault<T extends t.Any>(type: T, defaultValue: t.TypeOf<T>): t.Type<t.InputOf<T>, t.TypeOf<T>> {
  return new t.Type(
    `withDefault(${type.name}, ${JSON.stringify(defaultValue)})`,
    type.is,
    (v, c) => type.validate(v != null ? v : defaultValue, c),
    type.serialize
  )
}

const T1 = t.partial({
  name: withDefault(t.string, 'foo')
})

console.log(t.validate({}, T1).fold(() => 'error', a => JSON.stringify(a)))
// => `{}`

export function withDefault<T extends D.Decoder<unknown, unknown>>(
  decoder: T,
  defaultValue: D.TypeOf<T>
): D.Decoder<D.InputOf<T>, D.TypeOf<T>> {
  return D.union(
    decoder,
    pipe(
      undefinedDecoder,
      D.map(() => defaultValue)
    )
  );
}

export const undefinedDecoder = {
  decode: (val: unknown) => {
    return val === undefined
      ? D.success(undefined)
      : D.failure(val, "undefined");
  },
};

function withDefault<T extends t.Any>(type: T, defaultValue: t.TypeOf<T>): t.Type<t.TypeOf<T>> {
  return new t.Type(
    type.name,
    (v, c) => type.validate(v != null ? v : defaultValue, c)
  )
}