Typescript: Dynamically assigning array type based on preceding array element

Question

Typescript: Dynamically assigning array type based on preceding array element

Can Typescript support the implementation of an infinite Array (not a Tuple) with a type that depends on the previous element of the array?

Here is a sample pseudo-typescript code:

class B<T, U> {}

function foo<X, Y>(...args: [B<X, Z0>, B<Z0, Z1>, B<Z1, Z2>, ..., B<ZN, Y>]) {}

foo<string, number>(new B<string, number>, new B<number, boolean>, new B<boolean, number>); // Correct
foo<string, number>(new B<string, number, new B<number, boolean>); // Incorrect
foo<string, number>(new B<string, number>, new B<boolean, number>); // Incorrect

What should be substituted for "

[B<X, Z0>, B<Z0, Z1>, B<Z1, Z2>, ..., B<ZN, Y>]

" to make this function? Is it feasible at all?

Your insight is appreciated!

arrays typescript generics types

Answer 1

Answer №1

Recently, I was faced with a similar issue that required creating a lengthy chain of streams where the output of one stream had to be the input of the next. Here's how I tackled it:

type SkipFirst<T extends any[]> = T extends [any, ...infer R] ? R : never
type GetOrFallback<Object, Key, Default = never> = Key extends keyof Object ? Object[Key] : Default
type StreamChain<Start, End, Inputs extends [Start, ...any[]], Outputs extends any[] = SkipFirst<Inputs>> = {
    [I in keyof Inputs]: DuplexConstructor<Inputs[I], GetOrFallback<Outputs, I, End>>
}

export function createStreamChain<Initial, Final, Pipes extends [Initial, ...any[]]>(
    ...streamTypes: [ReadableConstructor<Initial>, ...StreamChain<Initial, Final, Pipes>, WritableConstructor<Final>]
): Writable {
    const source = streamTypes[0]
    const middleStreams = streamTypes.slice(1, -1) as StreamChain<Initial, Final, Pipes>
    const sink = streamTypes[streamTypes.length - 1] as WritableConstructor<Final>

    let head = readable(source)
    middleStreams.forEach((pipe) => (head = head.pipe(duplex(pipe))))
    return head.pipe(writable(sink))
}

Answer 2

Recently, I was faced with a similar issue that required creating a lengthy chain of streams where the output of one stream had to be the input of the next. Here's how I tackled it:

type SkipFirst<T extends any[]> = T extends [any, ...infer R] ? R : never
type GetOrFallback<Object, Key, Default = never> = Key extends keyof Object ? Object[Key] : Default
type StreamChain<Start, End, Inputs extends [Start, ...any[]], Outputs extends any[] = SkipFirst<Inputs>> = {
    [I in keyof Inputs]: DuplexConstructor<Inputs[I], GetOrFallback<Outputs, I, End>>
}

export function createStreamChain<Initial, Final, Pipes extends [Initial, ...any[]]>(
    ...streamTypes: [ReadableConstructor<Initial>, ...StreamChain<Initial, Final, Pipes>, WritableConstructor<Final>]
): Writable {
    const source = streamTypes[0]
    const middleStreams = streamTypes.slice(1, -1) as StreamChain<Initial, Final, Pipes>
    const sink = streamTypes[streamTypes.length - 1] as WritableConstructor<Final>

    let head = readable(source)
    middleStreams.forEach((pipe) => (head = head.pipe(duplex(pipe))))
    return head.pipe(writable(sink))
}

Typescript: Dynamically assigning array type based on preceding array element

Answer №1

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