0.4.0 TaskEither issues

[OliverJAsh]

Hey @gcanti, I'm upgrading to fp-ts 0.4.0 and I ran into this issue with TaskEither. See the error in the code.

Do you have any ideas what's going on here?

import * as apply from 'fp-ts/lib/Apply';
import * as tuple from 'fp-ts/lib/Tuple';
import { Task } from 'fp-ts/lib/Task';

import * as taskEither from './TaskEither';

import Tuple = tuple.Tuple;
import TaskEither = taskEither.TaskEither;

// tuple constructor
const toTuple2 = <A, B>(a: A, b: B): tuple.Tuple<A, B> => new tuple.Tuple([ a, b ]);

// lifted tuple constructor
const taskEitherToTuple2 = <L, A, B>(fa: TaskEither<L, A>, fb: TaskEither<L, B>) =>
    apply.liftA2(taskEither, (a: A) => (b: B) => toTuple2(a, b))(fa, fb);

type Tuple2TaskEither<L, A, B> = (tuple.Tuple<TaskEither<L, A>, TaskEither<L, B>>);

export const toTaskEitherTuple2 = <L, A, B>(t: Tuple2TaskEither<L, A, B>) =>
    taskEitherToTuple2(t.fst(), t.snd());

const taskEither1: TaskEither<number, number> = taskEither.of(1)
const taskEither2: TaskEither<number, number> = taskEither.of(2)
toTaskEitherTuple2(new Tuple([taskEither1, taskEither2]))
    // Error: [ts] Property 'chain' does not exist on type 'HKT<"TaskEither", Tuple<number, number>>'.
    .chain(tupleInstance => {

    })

[gcanti]

How is defined your TaskEither?

[OliverJAsh]

@gcanti I've just copied/pasted the lates one in the repo: https://github.com/gcanti/fp-ts/blob/260ab3a720bed237fef2d2c18761f71c3a1523b7/examples/TaskEither.ts

[gcanti]

Ok so, this fp-ts version is necessarily less smart than the previous one when it comes to transform a HKT<F, A> into a concrete type, it needs some hints through a list of "overloadings".

In this case liftA2 returns correctly HKT<"TaskEither", Tuple<number, number>> but while we, as developers, know that is equivalent to TaskEither<number, Tuple<number, number>> the compiler can't know that, so we need to add an hint

// add this to the `TaskEither` file

import { Curried2 } from 'fp-ts/lib/function'

declare module 'fp-ts/lib/Apply' {
  interface Ops {
    liftA2<A, B, C>(
      apply: Apply<URI>, // <= important: this is the URI of TaskEither
      f: Curried2<A, B, C>
    ): <L>(fa: TaskEither<L, A>, fb: TaskEither<L, B>) => TaskEither<L, C>
  }
}

In practice by doing that we provide a proof to the compiler that it can trasform a HKT<'TaskEither', A> into a TaskEither<L, A> in the context of liftA2. Does it make sense?

[OliverJAsh]

@gcanti Amazing, thank you. Good to understand why that is necessary.

Can we add this to the example TaskEither in this repo? I will try to write some documentation once I understand things a bit better.

[gcanti]

btw this is the reason why you can find a file named overloadings.ts in the lib folder which contains a big list of built-in overloadings written by me (PRs welcomed)