Flow type helpers

[fdecampredon]

Flow offer some nice type helpers in his recent versions, while some of them are already addressed by typescript, some could perhaps be helpful.
Here are extracted comments from flow source just for inspiration.

• $Either<...T> is the union of types ...T
• $All<...T> is the intersection of types ...T
• $Supertype acts as any over supertypes of T
• $Subtype acts as any over subtypes of T
• $Shape matches the shape of T
• $Diff<T,S>
• $Enum is the set of keys of T
• $Record is the type of objects whose keys are those of T

[RyanCavanaugh]

Sounds neat. It'd probably be best to log these separately if/when more detailed proposals become available.

[DanielRosenwasser]

I discussed with @wwwsevolod over Gitter - something like $ResultOf would be decently useful for the resulting type of calling a value of some type. This could return the union of types returned by all call signatures for a type.

[Lenne231]

There is another issue on this #3779

[benliddicott]

It seems like a $ResultOf equivalent functionality could be implemented by extending typeof to permit arbitrary expressions.

// E.g. func(1) returns {a:1}
function func<T>(a: T): { a: T } { return { a: a }; };

// Would like to say this:
// let a: typeof (func(1));

// But we can already say this:
let a = false ? func(1): void 0; // a is type {a: T}

Extending typeof to arbitrary expressions would provide more utility, as you could then also reference the types of instance members and return values:

class P { 
instanceProperty: {a: number, b: string};
}

let c: typeof((<P>(void 0)).instanceProperty.b) = "bval";

See also #4640 (comment) asking for something equivalent.

[s-panferov]

Hi all! Is this stuff is going to be implemented? Our teams is really interested in something like $Shape to improve ReactJS and Redux workflow.

[benliddicott]

Shape seems to be related to type guards. Is that correct?

[AlexGalays]

$Shape apears to be by far the highest in demand from that list. It's basically mandatory to have a typesafe Object.assign like construct.