Shouldn't the uppercase of a number be a string?

    const makeUpperCase = t.pattern(
      (input: string) => input.toUpperCase(),
      (input: number) => '' + input
    );

Otherwise you get odd behaviours like:

    makeUpperCase('1') + makeUpperCase('1') = '11' (probably what you want)
    makeUpperCase(1) + makeUpperCase(1) = 2 (probably not what you want)