Question regarding expected behavior of TE.fromTask

[niklaserik]

Hi,

I want to start to thank you for a great library!

My question is regarding the type of the natural transformation TaskEither's fromTask type. I have a chain of TaskEithers and hence I want to convert a my Task into a TaskEither to get a nice flow in the code. Since a Task never is supposed to fail, I expected the type after the transformation to be TE.TaskEither<never, T>. However, the transformation resulted in the type TE.TaskEither<unknown, T>

I can achieve the expected behavior by making an anonymous function like TE.chainW(flow(functionReturningTask, (a) => TE.fromTask<T, never>(a))) but it would be nice to just be able to write TE.chainW(flow(functionReturningTask, TE.fromTask ))

Perhaps there is a reason I an not aware about why it converts it to unknown´ instead of never`?

[gcanti]

@niklaserik that's how it should work, could you provide a repro?

[niklaserik]

@gcanti check the types of the anonymous functions in the dummy code below;

import { flow, pipe } from 'fp-ts/lib/function';
import * as T from 'fp-ts/Task';
import * as E from 'fp-ts/Either';
import * as TE from 'fp-ts/TaskEither';
import axios from 'axios';


const fetch = TE.tryCatch(()=>axios.get('https://google.com'),E.toError);
const task = (a: unknown):T.Task<void>  => ()=> Promise.resolve();
const a = pipe(
    fetch, 
    b=>b, 
    TE.chainW(flow(task, c=>c, TE.fromTask, d=>d )),
    f=>f,
);

Getting the same issue with unknown instead of never there.
I'm running it with "fp-ts": "2.12.2" and "typescript": "4.6.4"

[gcanti]

Looks like the culprit is using fromTask with flow.
Should be fixed in [email protected] (currently released as an RC)

However regardless of the version you are using, my advice would be to use the chainTaskK helper (basically chain + fromTask in one shot)

// const a: TE.TaskEither<Error, void>
const a = pipe(fetch, TE.chainTaskK(task))

[JoshuaAmaju]

How is chainTaskK different from chainTask. I've seen a lot of that variation for Option etc, what's the difference?

[niklaserik]

Thank you for the advice and the awesome library. Works perfectly.