Typelevel stuff in Flow

booleans.js

// Basic typelevel boolean functions.

type If<A: boolean, Then, Else = empty> = $Call<typeof If_, A, Then, Else>;

declare function If_<Then, Else>(true, Then, Else): Then;
declare function If_<Then, Else>(false, Then, Else): Else;

type Not<A: boolean> = If<A, false, true>;
type And<A: boolean, B: boolean> = If<A, B, false>;
type Or<A: boolean, B: boolean> = If<A, true, B>;
type Eq<A: boolean, B: boolean> = If<A, B, Not<B>>;

example-predicate.js

// Example type predicate.

type IsSimple<A> = $Call<typeof isSimple, A>;

declare function isSimple(string): true;
declare function isSimple(number): true;
declare function isSimple(boolean): true;
declare function isSimple(null): true;
declare function isSimple(void): true;
declare function isSimple(empty): true;
declare function isSimple(any): false;

// Example usage:

type A<T> = If<IsSimple<T>, { value: T }, T>;

example-recursion.js

// Example of pattern matching and recursion.

type CollectionType<A> = $Call<typeof collectionType, A>;

declare function collectionType<A>(Array<A>): CollectionType<A>;
declare function collectionType<A>(Set<A>): CollectionType<A>;
declare function collectionType<A>(Map<any, A>): CollectionType<A>;
declare function collectionType<A>(A): A;

// Example usage:

let a: CollectionType<Array<Set<Map<string, number>>>>; // number