[request] How to use the Reader pattern with Tasks

[brahimmouhamou]

Request

Would love some real live examples where using the Reader pattern makes your live easier.

I have a Jest example of a "hello world" example but it's a different thing to use it in combination with a combo of different types like TaskOption en TaskEithers.

Simple example:

import { pipe } from 'fp-ts/lib/function'
import * as R from 'fp-ts/Reader'

const input = 'Sup'

describe('reader test', () => {
	it('without Reader pattern', () => {
    // GIVEN
    const logger = (data: any) => console.log(data)
    const log = (logger: (data: string) => void) => (data: any) => logger(data)
    const addPrefix = (prefix: string) => (data: string) => `${prefix}${data}`
    const addExclamation = (isShouted: boolean) => (data: string) =>
      isShouted ? `${data.toUpperCase()}!!!` : data

    // Problem here is that 'start' needs to know all the dependencies
    // while using Reader they are infered.
    const start =
      ({
        logger,
        prefix,
        isShouted
      }: {
        logger: (data: string) => void
        prefix: string
        isShouted: boolean
      }) =>
      (data: string) =>
        pipe(data, addPrefix(prefix), addExclamation(isShouted), log(logger))

    // WHEN
    const result = start({ logger, prefix: 'wzz', isShouted: true })(input)

    // THEN
    // expect is not correct, should check the logs for the result
    expect(result).toBe(false)
  })


  it('with Reader pattern', () => {
    // GIVEN
    const logger = (data: any) => console.log(data)
    const log = (data: any) => ({ logger }: { logger: (data: string) => void }) => logger(data)
    const addPrefix = (data: string) => ({ prefix }: { prefix: string }) => `${prefix}${data}`
    const addExclamation = (data: string) => ({ isShouted }: { isShouted: boolean }) => 
			isShouted ? `${data.toUpperCase()}!!!` : data

		// As you can see here we do not need to define the dependencies when
		// creating our 'start' function
    const start = (data: string) =>
      pipe(R.of(data), R.chain(addPrefix), R.chainW(addExclamation), R.chainW(log))

    // WHEN
    const result = start(input)({ logger, prefix: 'wzz', isShouted: true })

    // THEN
    // expect is not correct, should check the logs for the result
    expect(result).toBe(false)
  })
})