Type-level conditions in Flow https://is.gd/OPsJBd

conditions.js

// Licensed under CC BY 4.0.

type $If<X: boolean, Then, Else = empty> = $Call<
    & ((true, Then, Else) => Then)
    & ((false, Then, Else) => Else),
    X,
    Then,
    Else,
>;

type $Not<X: boolean> = $If<X, false, true>;
type $And<X: boolean, Y: boolean> = $If<X, Y, false>;
type $Or<X: boolean, Y: boolean> = $If<X, true, Y>;

type $Gte<X, Y> = $Call<
    & ($Subtype<X> => true)
    & (mixed => false),
    Y,
>;

////

declare var a: $Gte<number, string>;

/* error  1 */ (a: true);
/* ok       */ (a: false);

declare var b: $Gte<number, number>;

/* ok       */ (b: true);
/* error  2 */ (b: false);

declare var c: $If<true, 1, 2>;

/* ok       */ (c: 1);
/* error  3 */ (c: 2);

declare var d: $If<false, 1, 2>;

/* error  4 */ (d: 1);
/* ok       */ (d: 2);

////

declare var e: $If<$Gte<number, string>, 1, 2>;

/* error  5 */ (e: 1);
/* ok       */ (e: 2);

declare var f: $If<$Gte<number, number>, 1, 2>;

/* ok       */ (f: 1);
/* error  6 */ (f: 2);

////

type IsNumber<X> = $Gte<number, X>;

type NumberTo12<X> = $If<IsNumber<X>, 1, 2>;

declare var g: NumberTo12<string>;

/* error  7 */ (g: 1);
/* ok       */ (g: 2);

declare var h: NumberTo12<number>;

/* ok       */ (h: 1);
/* error  8 */ (h: 2);

FWIW

declare var a2: $Gte<string, number>;
(a2: true);
(a2: false); // error

Fails in the same way. In other words, $Gte seems to always return true (as would be indicated by the $Subtype deprecation notice.)

if anyone is interested i have version of this without $Subtype

https://gist.github.com/thecotne/6e5969f4aaf8f253985ed36b30ac9fe0

$If seems to break when Then and Else are not a similar type and $If is instantiated with a boolean generic type parameter. Otherwise quite ingenious though I wish Flow would just have native support for conditional types like TypeScript does.