Flow silliness

peano.js

// @flow

// adapted from flown
type Is<A, B> = $Call<
  & (<A, B: A>(...args: [A, B]) => $Call<((B) => true) & (() => false), A>)
  & (() => false)
, A, B>;


// Peano arithmetic
class S<T> {}
// 0, S<0>, S<S<0>>, ...

type Inc<A> = S<A>;
type Dec<A> = $Call<<T>(S<T>) => T, A>;

type Add<A, B> = $Call<
  & ((0, B) => B)
  & (<T>(S<T>, B) => Add<T, S<B>>)
, A, B>;

type Zero = 0;
type One = S<Zero>;
type Two = S<One>;
type Three = S<Two>;

// Test Inc
(true:  Is<Three, Inc<Two>>);
(false: Is<Two, Inc<Two>>);
// Test Dec
(true:  Is<Two, Dec<Three>>);
(false: Is<Three, Dec<Three>>);
// Test Add
(true:  Is<Three, Add<One, Two>>);
(true:  Is<Three, Add<Two, One>>);
(false: Is<Three, Add<One, One>>);