feat request: allow preserving keys (+ types by key) to map over objects

[KiaraGrouwstra]

I'm hoping to be able to properly type functions like Ramda's R.map (for objects) or Lodash's _.mapValues -- in such a way as to acknowledge that mapped over values (which may have been of different types) may give results of different types, dependent on both their respective input type as well as on the mapper function.

Code

let arrayify = <T> (v: T) => [v];
declare function mapObject<T, V, M extends {[k: string]: T}>(func: (v: T) => V, m: M): {[K in keyof M]: V}
// ... or Lodash's _.mapValues, Ramda's R.map / R.mapObjIndexed / R.project...
mapObject({ a: 1, b: 'foo' }, arrayify)

Desired behavior:
{ a: number[], b: string[] }

Actual behavior:
{ a: any[], b: any[] }

I apologize for the use of libraries for this example. Probably more verbose without, but the concept is common in FP libraries.

[aluanhaddad]

This is an issue with the declaration files for ramda as they are specified in the linked repo. The new Mapped Types feature #12114 allows this pattern to be expressed very elegantly.

[KiaraGrouwstra]

I hadn't been aware, thank you for pointing this out! :)

[HerringtonDarkholme]

Hi, I think this might be relevant usage of delayed index access type for generic. More info here #11929 (comment).

Rambda's API is a perfect usage of that.

[KiaraGrouwstra]

If you'd forgive a follow-up question (which I can take to SO if deemed inappropriate here), I'm trying to figure out how to generalize the mapObj example of aluanhaddad's Mapped Types link:

function mapObject<K extends string | number, T, U>(obj: Record<K, T>, f: (x: T) => U): Record<K, U>;

... to enable types separated by key, while in this example we seem to have a single T/U for the whole (multi-key) function call.
So I get that this kind of separation could be achieved along the lines of example type T5 = { [P in keyof Item]: Item[P] };, but what's getting me stumped here is how to combine this with the generic function f: (x: T) => U. As in, I suppose U and T would at that point no longer have a single instance, but rather one per key. Would that still be possible?

[HerringtonDarkholme]

As in, I suppose U and T would at that point no longer have a single instance, but rather one per key.

I don't know whether it is possible in the long term. But it is impossible for now.

declare function mapObject<T, V, M extends {[k: string]: T}>(func: (v: T) => V, m: M): {[K in keyof M]: V} 

You can try this for now. It's the best of status quo.

[KiaraGrouwstra]

Thanks! That'll do for now then. :D
@mhegazy, if this could still be considered an open feature request, would you perhaps reconsider the question tag?

[mhegazy]

@mhegazy, if this could still be considered an open feature request, would you perhaps reconsider the question tag?

Mapped types should enable these scenarios. The change needed is to update the declaration files. if there are other ones that are not covered please provide more information about why mapped types is not sufficient.

[KiaraGrouwstra]

@mhegazy: I apologize, I'll admit my original example ended up not covering the breadth of my intended result. I've tried to update it accordingly.
The essence here is covered by the

U and T would no longer have a single instance, but rather one per key

.. which HerringtonDarkHolme stated is not presently possible.

Note that this becomes relevant in my (updated) example as different keys end up with different value types, dependent on both the transforming function (e.g. arrayify but could be anything) and the types of the input values.
Presently, input values are assumed to share the same type, and thus result types are only evaluated once (rather than per key) as well.

[mhegazy]

@ahejlsberg is working on adding inference from mapped types. so you should be able to write something like:

declare function map<T, K extends keyof T, U>(fn: (a: T) => U, obj: Record<K, T>): Record<K, U>;

[mhegazy]

actually scrap that. it was too late for me last night, and was not thinking right. this is not about generic inference from mapped types. this is about having the function somewhere in the template so that it applies to every property as it is being created. not sure how though..

but i guess we need is somehow apply the function for each value in the template:

declare function map<T>(fn: (a: T) => ?, obj: T): { [P in keyof T]: typeof f(T[P]) };

[KiaraGrouwstra]

Note that a solution here would also extend to allowing Array.prototype.map to be typed such as to handle heterogeneous arrays. Examples:

• Tuples (contents similar enough to share a mapper, distinct enough to make it desirable to keep them apart)
• Lists e.g. pets.map((pet: Pet) => pet instanceof Bird) could potentially return precise results on the type level already. Allowing results by item, where known, would better accommodate the fact that the typing system is becoming more granular.

[mhegazy]

Another issue here with the aggressive evaluation of the indexed access type is using another type parameter, for instance, trying to define Ramada.path:

declare function path<T, K1 extends keyof T, K2 extends keyof T[K1]>(keys: [K1, K2], obj: { [K1]: { [K2]: T } }): T;

`K2` has type `never` here, which is not useful. moreover, there is not way to make this work correctly

[KiaraGrouwstra]

@mhegazy: I tried to see if I could figure out a way to formulate map for tuples of length 2 as an easier version of this exercise: declare function map<A,B,T,U>(fn: (a: A) => B, tpl: [T,U]): [ typeof fn(T), typeof fn(U) ];
I'm not very confident about the requirements on typing fn here, particularly w.r.t. type constraints so as to guarantee T / U would match A. If the principle works though, I suppose a wall of typings with increasing numbers of generics could solve this for tuples, another version similar to your example hopefully for objects.

(If you have a good REPL for these things I'd be interested, right now I just tried a npm i -g on today's nightly of TypeScript, then tried testing in VSCode after setting this nightly as its TS version to use there, but it appeared to nevertheless see a syntax error in the function application.)

In your path example, maybe I can see what went wrong... you stated K1 extends keyof T, while you've defined T as the result, while it should rather be a key of the original object (or array technically). Perhaps it might make more sense like this?

declare function path<U, K1 extends keyof T, K2 extends keyof T[K1], T extends { [K1]: { [K2]: U } }>(keys: [K1, K2], obj: T): U;

If I try this in VSCode, on the K1/K2 (as used in the T definition) it gives me error squiggles 'K1' only refers to a type, but is being used as a value here. I'm still a bit stumped on that one.

If these can be addressed though, I'm hoping to solve the walls of 'definitions for ever-increasing numbers of generics' in my cross-linked reduce proposal.

[mhegazy]

@tycho01 these are not fixed yet, and this is what this issue is tracking.

also seems to be a duplicate of #12342

[KiaraGrouwstra]

I'll track that. Thank you. :)

[KiaraGrouwstra]

@mhegazy: I tried to think this over again, since most other TS issues I have could also be manually addressed by adding type annotations.

In your example, I'm a bit unfamiliar with the use of typeof at the type level, but what especially caught my eye in your example was your use of function application in the type language.
I've been under the impression this wasn't available in the current nightlies yet. This made me wonder, are you aware of any existing proposals to add this?
I'm now under the impression it could just suffice to address the problem here:

declare function map<T, F extends Function>(fn: F, obj: T): { [P in keyof T]: F(T[P]) };

(Note I referred to the function here using its type, F, rather than its name, fn, as you had. My rationale here was that its type should include the relevant information, while its name might be unavailable in certain contexts, e.g. if I wanted to write CurriedFunction2<F, T, { [P in keyof T]: F(T[P]) }>. Not sure it's a great example, but I hope it illustrates my line of thought.)

[mhegazy]

There is no way now to expresses the return type of a function. we have an issue #6606 tracking supporting this using typeof <expr>.