Perform side effect and return original input (tee)

[abacaj]

📖 Documentation

Does fp-ts have a concept of a tee example: https://davefancher.com/2015/01/11/extending-f-pipelines-with-a-tee-function/

“given a value, apply a function to it, ignore the result, then return the original value."

I've got a use case for TaskEither to perform some uploading and return the original input to the next pipeline function.

[abacaj]

example use case:

pipe(
  fileUrls,
  TaskEither.tee(upload),
  TaskEither.chain(parseCsv)
)

[abacaj]

This seems to work for my use case:

export const taskEitherEffect = <E, A>(f: (a: A) => TaskEither<E, void>) => (b: TaskEither<E, A>) => {
    return pipe(
        b,
        TE.fold((e) => {
            return TE.left(e);
        }, (value) => {
            return pipe(
                f(value),
                TE.map(() => value)
            );
        })
    )
}

usage:

pipe(
  fileUrls,
  taskEitherEffect(upload)
  TaskEither.chain(parseCsv)
)

Upload just returns a promise with an either (taskeither) error and void

upload: (fileUrls: string[]) => TaskEither<Error, void>

The parseCsv now has access to the original input (fileUrls) and if the upload fails we pass the error along instead of the original input.

[gcanti]

You may want chainFirst

import { pipe } from 'fp-ts/lib/pipeable'
import * as TE from 'fp-ts/lib/TaskEither'

declare const fileUrls: Array<string>
declare function upload(fileUrls: Array<string>): TE.TaskEither<Error, void>
declare function parseCsv(fileUrls: Array<string>): TE.TaskEither<Error, void>

pipe(TE.right(fileUrls), TE.chainFirst(upload), TE.chain(parseCsv))

[gkamperis]

@gcanti would be great to add this to the docs and a recipes/how to page...

[abacaj]

You may want chainFirst

import { pipe } from 'fp-ts/lib/pipeable'
import * as TE from 'fp-ts/lib/TaskEither'

declare const fileUrls: Array<string>
declare function upload(fileUrls: Array<string>): TE.TaskEither<Error, void>
declare function parseCsv(fileUrls: Array<string>): TE.TaskEither<Error, void>

pipe(TE.right(fileUrls), TE.chainFirst(upload), TE.chain(parseCsv))

Yep, that worked - it's a direct replacement for my implementation. Thanks!

[gcanti]

@gkamperis any contribution to the documentation is welcomed!

[gkamperis]

@gcanti of course...
however I do not feel I have enough knowledge to create these sort of examples and I do not know if this page already exists or where it should live.

Is there an example I can look at?

[JohnForster]

Just a quick question on this, what would be the correct way to implement this if you also didn't care if the upload failed or not? I'd like to be able to discard the Error state as well. Is there a better way to implement this?

[mohaalak]

Just a quick question on this, what would be the correct way to implement this if you also didn't care if the upload failed or not? I'd like to be able to discard the Error state as well. Is there a better way to implement this?

I think you can use TaskEither.getOrElse to get a Task

[gcanti]

you could define a discard utility:

const discard: <E>(ma: TE.TaskEither<E, void>) => TE.TaskEither<E, void> = TE.alt(() =>
  TE.right(undefined as void)
)

const result = pipe(TE.right(fileUrls), TE.chainFirst(flow(upload, discard)), TE.chain(parseCsv))

in fp-ts@2.10 (<= currently a release candidate) there's a chainFirstTaskK utility:

const result = pipe(TE.right(fileUrls), TE.chainFirstTaskK(upload), TE.chain(parseCsv))