How would you classify a function that accepts an input of T<A>[] and returns T<A[]>?

Question

How would you classify a function that accepts an input of T<A>[] and returns T<A[]>?

Here are a few instances

const f = <L, R>(xs: Either<L, R>[]): Either<L, R[]> => { throw new Error('Not Implemented') };

const f = <T>(xs: Promise<T>[]): Promise<T[]> => { throw new Error('Not Implemented') };

const f = <T>(xs: Box<T>[]): Box<T[]> => { throw new Error('Not Implemented') };

All of these involve some form of reduction. However, the semigroup being used is simply a List.

When it comes to readability, what would be appropriate names for such functions?

There is an element of subjectivity here which is discouraged on this forum. Nonetheless, it could also be argued that a name can be objectively categorized as bad or good based on certain criteria/reasoning.

Perhaps something like collect or gather along those lines?

typescript functional-programming naming-conventions naming

Answer 1

Answer №1

While TypeScript may not be Haskell, it's often my go-to place to look for existing names for the type of object I'm developing.

If we consider Either<L, T>, Promise<T>, and Box<T> all as applicative functors over T, then functions like

<L,T>(x: Array<Either<L, T>>) => Either<L, Array<T>>

,

<T>(x: Array<Promise<T>>) => Promise<Array<T>>

, and

<T>(x: Array<Box<T>>) => Box<Array<T>>

are likely to be called sequence.

The criteria for a type function F<T> being an applicative functor over T could be outlined by having certain functions that behave in this way:

declare function pure<T>(x: T): F<T>;
declare function lift2<A, B, T>(cb: (a: A, b: B) => T): (fa: F<A>, fb: F<B>) => F<T>;

Using the above guidelines, an implementation of sequence could look like this:

function sequence<T>(x: Array<F<T>>): F<Array<T>> {
    return x.reduce(lift2<T[], T, T[]>((xs, x) => xs.concat(x)), pure([]))
}

For Promise<T>, the implementation is quite straightforward:

function pure<T>(x: T): Promise<T> {
    return Promise.resolve(x);
}
function lift2<A, B, T>(cb: (a: A, b: B) => T): (fa: Promise<A>, fb: Promise<B>) => Promise<T> {
    return (fa: Promise<A>, fb: Promise<B>) => fa.then(a => fb.then(b => cb(a, b)))
}

In a similar fashion, implementations could be devised for Either<L, T> and Box<T>.

I hope this information proves useful for you. Best of luck!

Link to code

Answer 2

While TypeScript may not be Haskell, it's often my go-to place to look for existing names for the type of object I'm developing.

If we consider Either<L, T>, Promise<T>, and Box<T> all as applicative functors over T, then functions like

<L,T>(x: Array<Either<L, T>>) => Either<L, Array<T>>

,

<T>(x: Array<Promise<T>>) => Promise<Array<T>>

, and

<T>(x: Array<Box<T>>) => Box<Array<T>>

are likely to be called sequence.

The criteria for a type function F<T> being an applicative functor over T could be outlined by having certain functions that behave in this way:

declare function pure<T>(x: T): F<T>;
declare function lift2<A, B, T>(cb: (a: A, b: B) => T): (fa: F<A>, fb: F<B>) => F<T>;

Using the above guidelines, an implementation of sequence could look like this:

function sequence<T>(x: Array<F<T>>): F<Array<T>> {
    return x.reduce(lift2<T[], T, T[]>((xs, x) => xs.concat(x)), pure([]))
}

For Promise<T>, the implementation is quite straightforward:

function pure<T>(x: T): Promise<T> {
    return Promise.resolve(x);
}
function lift2<A, B, T>(cb: (a: A, b: B) => T): (fa: Promise<A>, fb: Promise<B>) => Promise<T> {
    return (fa: Promise<A>, fb: Promise<B>) => fa.then(a => fb.then(b => cb(a, b)))
}

In a similar fashion, implementations could be devised for Either<L, T> and Box<T>.

I hope this information proves useful for you. Best of luck!

Link to code

How would you classify a function that accepts an input of T<A>[] and returns T<A[]>?

Answer №1

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