Added in some of the great info about non-homomorphic transformations…

[JakeGinnivan]

… in type mapping from microsoft/TypeScript#13224 (comment)

Just trying to extend the docs for something I could not figure out how to do after reading the docs on type mapping. // @mhegazy

[mhegazy]

@sandersn can you take a look

[sandersn]

I think the discussion should be broken up and moved toward the end of the mapped type section since it's an advanced concept.

[sandersn]

This is too early for this explanation, and it's too dense for the handbook. It would be better to weave it in with the "Real applications" discussion below.

[sandersn]

Homomorphic mapped types are already described briefly in the "Inference from mapped types" section, so that section needs to be updated as well.

[sandersn]

I like this example but its description of purpose is confusing.

[sandersn]

Need an example of coping vs not-copying.

[sandersn]

I think my requests amount to rewriting the whole PR, so I made a PR at #473 that does more or less that. @JakeGinnivan @mhegazy do you want to take a look?

I like the EnsureAll example but I ended up using Pick and Record as examples since they were already discussed as part of the built-in types.

[JakeGinnivan]

Sounds good to me @sandersn, was just an area where I had some difficulty making it work the way I wanted.

[masaeedu]

Did you actually test this EnsureAll type? I get a type error saying you can't index T with K, presumably because K isn't constrained to the keys of T.

[masaeedu]

@sandersn Could you please add an example for EnsureAll as well? It isn't clear how to do the reverse transformation of Partial, and I suspect it might not be possible in the current type system.

I'm testing the following with 2.2.2, and can't get it to work:

type Required<T, K extends keyof T> = {
    [P in K]: T[P]
};

interface X {
    y?: string 
}

let safeX: Required<X, keyof X> = {}; // Assignment should have failed
let y: string = safeX.y;              // Error: string | undefined is not assignable to string