Define array type where the type of each element relates to the type of the previous element

[tidal marlin]

Hi, I want to define an array type which would be the logical extension of the following, but for arrays of arbitrary length.

type Func<X,Y> = (obj:X) => Y;
type Arr<A,B,C,D,E,...> = [Func<A,B>,Func<B,C>,Func<C,D>,Func<D,E>...]

Regardless of whether this is the best solution to my specific problem, I am curious if typescript supports this kind of type.

[rugged cloak]

you can do it using recursion over a tuple

[main herald]

that_guy977#0

Playground Link

Preview:```ts
type Func<X, Y> = (obj: X) => Y
type Arr<T extends readonly unknown[]> = T extends [
infer First,
infer Second,
...infer Rest
]
? [Func<First, Second>, ...Arr<[Second, ...Rest]>]
: []

type X = Arr<[number, string, number, bigint, boolean]>
// ^?```

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[rugged cloak]

something like this

[tidal marlin]

Excellent. Thanks for that, I'll have a play

[main herald]

that_guy977#0

Playground Link

Preview:ts type Func<X, Y> = (obj: X) => Y type Arr<T extends readonly unknown[]> = T extends [ infer First, infer Second, ...infer Rest ] ? [Func<First, Second>, ...Arr<[Second, ...Rest]>] : T extends [infer First, ...infer Rest] ...

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[rugged cloak]

this absolute behemoth also supports variable length inputs

[tidal marlin]

hmm, actually, I think I'm still stuck

How to explain my conundrum...

type Getter<X,Y> = (obj:X) => Y;

function member<O extends {}, M extends keyof O>(m: M): Getter<O, O[M]> {
    return (obj: O) => obj[m];
}
function element<A extends Array<unknown>, E extends keyof A & number>(e: E): Getter<A, A[E]> {
    return (arr: A) => arr[e];
}

function get<X,Y>(obj: X, ...getters: [...]): Y {

}

the implementation would just chain the getters together to get the final value

I could easily do it with nested getters, but the ergenomics get ugly

I dont want something like `member(element(member(member(obj, 'a'), 'b'), 1, 'c')

I would prefer something like get(obj, member('a'), member('b'), element(1), member('c'))

[rugged cloak]

why can't you just do obj.a.b[1].c

[tidal marlin]

The example is contrived

[rugged cloak]

fair enough

[tidal marlin]

the real problem is more complex

[rugged cloak]

i think a mapped tuple would do better maybe?

[tidal marlin]

Its also something of an interesting problem to me 🙂

mapped tuple, like what you did or something else?

[rugged cloak]

my example doesn't have mapped tuples

one sec

i think this might need to be recursive for type safety

[tidal marlin]

oh yeah, sorry, I've not done any advanced typescript for like a year which is why im struggling

[rugged cloak]

i have no idea what's going on here

well fyi i guess, mapped tuples are written like mapped types and let you.. well, map tuples

[tidal marlin]

oh, I know what they are. I just confused the geneal term "mapping" with the specific typescript term of mapped tuples

not to say that I'm great at using them though. more I have used on occasion and know they exist

[tidal marlin]

Looks like I can solve it reasonably by only allowing a finite maximum number of elements

a bit yuckky, but quite workable

function get<A>(obj: A): A;
function get<A,B>(obj: A, ...getters: [Getter<A, B>]): B;
function get<A,B,C>(obj: A, ...getters: [Getter<A, B>, Getter<B, C>]): C;
function get<A,B,C,D>(obj: A, ...getters: [Getter<A, B>, Getter<B, C>, Getter<C, D>]): D;
function get<A,B,C,D,E>(obj: A, ...getters: [Getter<A, B>, Getter<B, C>, Getter<C, D>, Getter<C, E>]): E;

function get<A>(obj: A, ...getters: Array<Getter<unknown, unknown>>): unknown {
    let result: any = obj;
    for (const getter of getters)
        result = getter(result);
    return result;
}

[mild moon]

@tidal marlin look like more of a chained list than an array to me