Mixing Either and TaskEither in a pipe in fp-ts

Question

I have the following program that works fine when none of the functions is async.

interface Product {
  count: number
  pricePerItem: number
}

interface Tax {
  tax: number
}

interface Delivery {
  delivery: number
}

interface PTD { //ProductTaxDelivery
  p: Product
  t: number
  d: number
}

function getProduct(): Either<Error, Product> {
  return E.right({ count: 10, pricePerItem: 5 })
}

function getTax(p: Product): Either<Error, number> {
  return E.right(p.pricePerItem * p.count * 0.085)
}

function getDelivery(p: Product): Either<Error, number> {
  return  E.right(p.count * 0.05)
  //or maybe return E.left(Error('some error in delivery happened'))
}
function run(): Either<Error, PTD> {
  return pipe(
    E.Do,
    E.bind('p', getProduct),
    E.bind('tax', ({p}) => getTax(p)),
    E.bind('delivery', ({p}) => getDelivery(p)),    
    E.map(({ p, tax, delivery }) => ({ p, t: tax, d: delivery }))
  )
}
function main() {
  pipe(
    run(),
    E.fold(
      (e) => {
        console.log(`error: ${e}`)
      },
      (it) => {
        console.log(`ok ${it.p.count} ${it.p.pricePerItem} ${it.t} ${it.d}`)
      }
    )
  )
}

main()

The question I'm having is if one of my functions, for example getDelivery() is async, then I'm not sure how to solve it.

Here's what I have tried:

function getDelivery(p: Product): TaskEither<Error, number> {
  return TE.right(p.count * 0.05)
}

TE.bind('delivery', ({p}) => getDelivery(p)),

and many other variations, but all ended up in compiler errors.

The equivalent in imperative style is something like:

const getDelivery = async (p: Product) => {
   return await something()
}

const run = async (): PTD => {
   const product = getProduct()
   const tax = getTax(product)
   const delivery = await getDelivery(product)
   
   return {
      p: product, t: tax, d: delivery
   }
}

What is the correct functional way (that I think involves both Either and TaskEither) using fp-ts?

Update: I also tried to replace Either with TaskEither, E with TE everywhere, but the problem is now a compiler error when I tried to fold in main(). Here's the code that replaces:

function getProduct(): TaskEither<Error, Product> {
  return TE.right({ count: 10, pricePerItem: 5 })
}

function getTax(p: Product): TaskEither<Error, number> {
  return TE.right(p.pricePerItem * p.count * 0.085)
}

function getDelivery(p: Product): TaskEither<Error, number> {
  return TE.right(p.count * 0.05)
}

function run(): TaskEither<Error, PTD> {
  return pipe(
    TE.Do,
    TE.bind('p', getProduct),
    TE.bind('tax', ({ p }) => getTax(p)),
    TE.bind('delivery', ({ p }) => getDelivery(p)),
    TE.map(({ p, tax, delivery }) => ({ p, t: tax, d: delivery }))
  )
}

function main() {
  pipe(
    run(),
    TE.fold(
      (e) => { 
        console.log(`error: ${e}`)
      },
      (it) => {
        console.log(`ok ${it.p.count} ${it.p.pricePerItem} ${it.t} ${it.d}`)
        //doNonFunctional()
      }
    )
  )
}

main()

On line with (e) => {, the compiler error says:

error TS2345: Argument of type '(e: Error) => void' is not assignable to parameter of type '(e: Error) => Task<unknown>'.
  Type 'void' is not assignable to type 'Task<unknown>'.

Update 2 OK, so I get the code to compile but no output when the program runs

const printError = (e: Error): T.Task<unknown> => {
  console.log(`error: ${e}`)
  return () => Promise.resolve()
}

const printPTD = (ptd: PTD): T.Task<unknown> => {
  console.log(`ok ${ptd.p.count} ${ptd.p.pricePerItem} ${ptd.t} ${ptd.d}`)
  return () => Promise.resolve()
}

function run(): TaskEither<Error, PTD> {
  return pipe(
    TE.Do,
    TE.bind('p', getProduct),
    TE.bind('tax', ({ p }) => getTax(p)),
    TE.bind('delivery', ({ p }) => getDelivery(p)),
    TE.map(({ p, tax, delivery }) => ({ p, t: tax, d: delivery }))
  )
}

function main() {
  pipe(
    run(),
    TE.fold(
      (e) => printError(e),
      (ptd) => printPTD(ptd)      
    )
  )
}

main()

Answer 1

The issue is when you create a Task in main with pipe, you are not actually running anything.

This is how Task is defined:

interface Task<A> {
  (): Promise<A>
}

// same as type Task<A> = () => Promise<A>

Because Task is a thunk, you need to call it to actually execute the code.

async function main(): Promise<void> {
  await pipe(
    // ...
// vv note the call here
  )()
}

main()

However, I would do it like this:

const main: T.Task<void> = pipe(/* ... */)

main()

Similarly, run doesn't need to be a function; it can be const run = pipe(/* ... */).

Also, there's a Console module that provides log functions that return an IO (a type for side-effectful actions).

Your code could be written as

import * as Console from 'fp-ts/Console'
import * as E from 'fp-ts/Either'
import * as T from 'fp-ts/Task'
import * as TE from 'fp-ts/TaskEither'
import {pipe} from 'fp-ts/function'

// <A>(a: A) => Task<void>
const taskLog = T.fromIOK(Console.log)

// You can still keep getProduct and getTask synchronous
function getProduct(): E.Either<Error, Product> { /* ... */ }
function getTax(p: Product): E.Either<Error, number> { /* ... */ }

function getDelivery(p: Product): TE.TaskEither<Error, number> { /* ... */ }

const run: TE.TaskEither<Error, PTD> = pipe(
  TE.Do,
  // See below for what TE.fromEither(K) does
  TE.bind('p', TE.fromEitherK(getProduct)),
  TE.bind('tax', ({p}) => TE.fromEither(getTax(p))),
  TE.bind('delivery', ({p}) => getDelivery(p)),
  TE.map(({p, tax, delivery}) => ({p, t: tax, d: delivery}))
)

const main: T.Task<void> = pipe(
  run,
  TE.fold(
    e => taskLog(`error: ${e}`),
    it => taskLog(`ok ${it.p.count} ${it.p.pricePerItem} ${it.t} ${it.d}`)
  )
)

main().catch(console.error)

TE.fromEither converts an Either into a TaskEither:

export declare const fromEither: NaturalTransformation22<'Either', 'TaskEither'>
// same as
export declare const fromEither: <E, A>(fa: Either<E, A>) => TaskEither<E, A>

TE.fromEitherK is the same as fromEither but for functions:

export declare const fromEitherK: <E, A extends readonly unknown[], B>(f: (...a: A) => Either<E, B>) => (...a: A) => TaskEither<E, B>

You can probably guess by now what T.fromIOK (used for taskLog) does:

export declare const fromIOK: <A, B>(f: (...a: A) => IO<B>) => (...a: A) => Task<B>

Here's a CodeSandbox with the full code.