How should I use Writer?

[timongh]

Are there any examples of how to use Writer?

[THOUSAND-SKY]

I would myself like an example of this:

import Control.Monad.Writer  
  
logNumber :: Int -> Writer [String] Int  
logNumber x = Writer (x, ["Got number: " ++ show x])  
  
multWithLog :: Writer [String] Int  
multWithLog = do  
    a <- logNumber 3  
    b <- logNumber 5  
    return (a*b)  

(from http://learnyouahaskell.com/for-a-few-monads-more#writer ) Although I realize that example is outdated (see https://stackoverflow.com/questions/11684321/how-to-play-with-control-monad-writer-in-haskell )

[gcanti]

Rough translation

import * as C from 'fp-ts/Chain'
import { pipe } from 'fp-ts/function'
import * as RA from 'fp-ts/ReadonlyArray'
import * as W from 'fp-ts/Writer'

const Monad = W.getMonad(RA.getMonoid<string>())
const bind = C.bind(Monad)
const Do = Monad.of({})

const logNumber =
  (n: number): W.Writer<ReadonlyArray<string>, number> =>
  () =>
    [n, ['Got number: ' + n]]

const multWithLog = pipe(
  Do,
  bind('a', () => logNumber(3)),
  bind('b', () => logNumber(5)),
  W.map(({ a, b }) => a * b)
)

console.log(multWithLog()) // => [ 15, [ 'Got number: 3', 'Got number: 5' ] ]