Chain fp-ts TaskEither with Either in right

Question

I have a flow of 2 nested request, where could be 3 different results:

One of requests return Error
User is not Anonymous, return Profile
User is Anonymous, return false

Both requests could throw an error, and becaues of that implements TaskEither

const isAuth = ():TE.TaskEither<Error, E.Either<true, false>>  
   => TE.tryCatch(() => Promise(...), E.toError)
const getProfile = ():TE.TaskEither<Error, Profile>  
   => TE.tryCatch(() => Promise(...), E.toError)

The first request returns the boolean status of user authorization. Second request loads user profile if the user is authorized.

In return, I want to get the next signature, Error or Either with Anonymous/Profile:

E.Either<Error, E.Either<false, Profile>>

I tried to make it this way:

pipe(
    isAuth()
    TE.chain(item => pipe(
      TE.fromEither(item),
      TE.mapLeft(() => Error('Anonimous')),
      TE.chain(getProfile)
    ))
  )

But in return, I get E.Either<Error, Profile>, witch not convenient because I have to extract Anonymous status by hands from Error.

How to solve that question?

`E.Either` doesn't make much sense because the type is `Either`, so you cannot get an `Either` anyway. The reason why you lose the `Either` layer is the natural transformation `TE.fromEither(item)` from `Either` to `Task` you perform as the first step of your composition. — , May 02 '20 at 13:42
Oops, just noticed that your initial Either has literals as type parametrs and hence is `Either`. The Issue remains the same though. — , May 02 '20 at 13:45
@bob yes, I user natural transformation to make it work, because I don't find a way to write correct pipe, that will return `E.Either` or `Option`, which is not a point, in the right part, and put Error from the second request to Left part - it's what my question about. — Ivan Tarasov, May 02 '20 at 16:02

MnZrK · Accepted Answer · 2020-05-03T20:06:52.810

Don't know if you oversimplify actual code, but E.Either<true, false> is isomorphic to just boolean, so let's stick with simpler thing.

declare const isAuth: () => TE.TaskEither<Error, boolean>;
declare const getProfile: () => TE.TaskEither<Error, Profile>;

Then you add condition branch based on whether its authed or not and wrap the result of getProfile:

pipe(
  isAuth(),
  TE.chain(authed => authed 
    ? pipe(getProfile(), TE.map(E.right)) // wrap the returned value of `getProfile` in `Either` inside the `TaskEither`
    : TE.right(E.left(false))
  )
)

This expression has type TaskEither<Error, Either<false, Profile>>. You probably need to add some type annotations for it to typecheck properly, I haven't run the code myself.

EDIT:

You probably need to extract lambda as named function to get proper typings, something like so:

const tryGetProfile: (authed: boolean) => TE.TaskEither<Error, E.Either<false, Profile>> = authed
  ? pipe(getProfile(), TE.map(E.right))
  : TE.right(E.left(false));

const result: TE.TaskEither<Error, E.Either<false, Profile>> = pipe(
  isAuth(),
  TE.chain(tryGetProfile)
);

It works thank you! But how to get the correct types? I tried this `A.sequenceT(TE.taskEitherSeq)(result)`, but in return, I get Array type `TE.TaskEither]>`, which is not what I am looking for :( Do you know how to solve that? — Ivan Tarasov, May 03 '20 at 08:26
@IvanTarasov Not exactly sure, maybe there is an error or a typo. I updated the answer with some typing suggestions, you can work from there, it should give you some sensible compiler errors. — MnZrK, May 03 '20 at 20:08

Chain fp-ts TaskEither with Either in right

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