How to flatten TaskEither<Error, Either<Error, T>> in FP-TS

Issue

I am new to fp-ts. I am used to scala for comprehensions when dealing with combining Either, IO and ... The question is, let's say we have function with signatures

either(T): Either<Error, T>
ioEither(T): IOEither<Error, T>
taskEither(T): TaskEither<Error, T>
...

And I want to combine them, and I get something along the lines of

pipe(T, taskEither, map(either)): TaskEither<Error, Either<Error, T>>

Which should be flattened.

How to combine function of types T -> M<T> where M is IO or Task, and T -> M<Error, T> where M is Either, TaskEither or IOEither, flattening in process, like with scala for comprehensions?

Solution

One way to go about this is to simply pick TaskEither as your monad of choice, and lift everything else into it.

import { pipe } from 'fp-ts/function'
import { Either } from 'fp-ts/Either'
import { IOEither } from 'fp-ts/IOEither'
import { TaskEither, fromEither, fromIOEither, chain } from 'fp-ts/TaskEither'

declare const either: <T>(t: T) => Either<Error, T>
declare const ioEither: <T>(t: T) => IOEither<Error, T>
declare const taskEither: <T>(t: T) => TaskEither<Error, T>

const liftedIOEither = <T>(t: T) => fromIOEither(ioEither(t))
const liftedEither = <T>(t: T) => fromEither(either(t))

const res = pipe(
  1,
  liftedEither,
  chain(liftedIOEither),
  chain(taskEither),
) // TaskEither<Error, number>

Answered By - bugs