Async functions that return TaskEither

[nikkonrom]

I'm trying to refactor my functions that return Promise<Either<Error, Result>> to functions that return TaskEither<Error, Result>>.

For example, I have such function:

public async resolveSmaIds(
    explorationRequest: ExplorationRequest
  ): Promise<Either<ExternalExplorationError.ExternalExplorationProviderUnavailable, SmaId[]>> {

  // ...
  // some data preparations with await statement
  // ...

  return this.externalExplorationProvider.exploreDiscovery(
          explorationRequest.requestData as DiscoveryRequestDto,
          explorationRequest.hash
        ); // async call to external service that also returns Promise<Either<ExternalExplorationError.ExternalExplorationProviderUnavailable, SmaId[]>>
}

Then i'm trying to refactor it for returning TaskEither instead Promise<Either< ... , ... >>:

public async resolveSmaIds(
    explorationRequest: ExplorationRequest
  ): TaskEither<ExternalExplorationError.ExternalExplorationProviderUnavailable, SmaId[]>
{
// same body
}

Regardless of the return value, I'm getting TS compiler error:

TS1064: The return type of an async function or method must be the global Promise type. Did you mean to write 'Promise >'?

If I drop the async modifier from my function this error disappears, but now I can't use await statement anymore.

TS1308: 'await' expressions are only allowed within async functions and at the top levels of modules.

Is this mean that I need to get rid of all async functions in the project and refactor them to TaskEither?

[gcanti]

but now I can't use await statement anymore

you shouldn't need to use await in most cases, you can use map and chain instead:

import { pipe } from 'fp-ts/function'
import * as TE from 'fp-ts/TaskEither'

// -------------------------------------------------------------------------------------
// promise-based
// -------------------------------------------------------------------------------------

const promiseBasedAPI = async (s: string): Promise<number> => Promise.resolve(s.length)

const promiseBasedProgram = async (s: string): Promise<boolean> => {
  const n = await promiseBasedAPI(s)
  return n > 0
}

// -------------------------------------------------------------------------------------
// task-based
// -------------------------------------------------------------------------------------

const taskBasedAPI = TE.tryCatchK(promiseBasedAPI, (reason) => new Error(`${reason}`))

const taskBasedProgram = (s: string) =>
  pipe(
    taskBasedAPI(s),
    TE.map((n) => n > 0)
  )

[nikkonrom]

Ok, now it seems to be clear, thanks.