Fp-ts: Question on using flow.

[rikbrowning]

Question

Current Behavior

I can't understand why the following code doesn't work. The types all seem to be correct

const likeArticle = flow(
  api.likeArticle,
  setArticleLike,
  chain(updateLikedArticlesCount)
);

The above functions have the following signatures:

likeArticle: (id?: number | undefined) => TaskEither<undefined, number>
const setArticleLike: (ma: TaskEither<undefined, number>) => TaskEither<undefined, Article>
chain<undefined, Article, Article>(f: (a: Article) => TaskEither<undefined, Article>): (ma: TaskEither<undefined, Article>) => TaskEither<undefined, Article>
updateLikedArticlesCount: (article: Article) => TaskEither<undefined, Article>

I am trying to call it as likeArticle(1) for example but all it returns is a chain function rather than a TaskEither.

I can use pipe as follows:

const likeArticle = (id?: number) =>
  pipe(api.likeArticle(id), setArticleLike, chain(updateLikedArticlesCount))();

But seems like I should be able to pass id into the pipe someway rather than needing this outside wrapping function.

[gcanti]

@rikbrowning FYI you can use declare in order to write a repro:

import { flow } from 'fp-ts/lib/function'
import { chain, TaskEither } from 'fp-ts/lib/TaskEither'

type Article = string

declare const likeArticle: (id?: number) => TaskEither<undefined, number>
declare const setArticleLike: (ma: TaskEither<undefined, number>) => TaskEither<undefined, Article>
declare const updateLikedArticlesCount: (article: Article) => TaskEither<undefined, Article>

// program: (id?: number) => TaskEither<undefined, string>
const program = flow(likeArticle, setArticleLike, chain(updateLikedArticlesCount))

[rikbrowning]

@gcanti TIL, that's really cool. Sorry you'll have to forgive my ignorance

[gcanti]

@rikbrowning no problem, that's kind of a hack but it works well.

Back to this issue, can we close it? looks like program has the expected type.

[rikbrowning]

@gcanti it has the expected type but when I execute it I get back function chain rather than a TaskEither?

[gcanti]

Weird, if I add the implementations I can see the expected result

import { flow } from 'fp-ts/lib/function'
import { chain, right, TaskEither } from 'fp-ts/lib/TaskEither'

type Article = string

const likeArticle = (_id?: number): TaskEither<undefined, number> => right(1)
const setArticleLike = (_ma: TaskEither<undefined, number>): TaskEither<undefined, Article> => right('a')
const updateLikedArticlesCount = (article: Article): TaskEither<undefined, Article> => right(article)

flow(likeArticle, setArticleLike, chain(updateLikedArticlesCount))(1)().then(console.log)
// { _tag: 'Right', right: 'a' }

[rikbrowning]

flow(likeArticle, setArticleLike, chain(updateLikedArticlesCount))(1)() is that not the equivalent of program(1)()? Is there no way to have the call behave as program(1) without extra function call? Ideally I want to expose the flow function to accept the ID without having to the wrapper.

[rikbrowning]

After some more reading, is the extra () required to execute the TaskEither?

[gcanti]

is the extra () required to execute the TaskEither?

Yes if you want to get back a promise, TaskEither<E, A> is just an alias of () => Promise<Either<E, A>>

[rikbrowning]

Awesome, that's the part that was confusing me

[steida]

It should be explained in the documentation why IO, Task, TaskEither need to be called. Something like: Side-effect composition can be called now or later. In FP, we split definition from execution. Am I correct?