Discussion on: Getting started with fp-ts: IO

[Giulio Canti]

You return a (representation of a) side effect rather than execute a side effect.

Example

import { log } from 'fp-ts/lib/Console'
import { IO, io } from 'fp-ts/lib/IO'
import { chain, fold, none, Option, some } from 'fp-ts/lib/Option'
import { pipe } from 'fp-ts/lib/pipeable'

function getCharactersA(s: string): Option<string> {
  return s.length > 3 ? some(s.substring(3)) : none
}

function program(input: string): IO<void> {
  return pipe(
    some(input),
    chain(getCharactersA),
    // return my side effect only if there is a value
    fold(() => io.of(undefined), log)
  )
}

program('foo')() // no output
program('barbaz')() // outputs "baz" to the console

[Daniel Steigerwald]

Or fold(io.of(constVoid))