A Quick Lap with the Writer Monad

Introduction

The Writer monad allows functions to accumulate information as functions execute. According to the Hackage page:

A writer monad parameterized by the type w of output to accumulate.

Perhaps not the most verbose of descriptions, however this is rather simple to explain with a well known example. In previous programming disciplines you would have needed to log information out of your code as your program “did things”. The Writer monad allows you to write out information in a log form. This doesn’t necessarily have to be in textual log format; an example I have seen recently is to keep track of calculations used to come to a final result. The calculations put into that log sped up calculations on other figures.

The idea here is to not clutter your code having to support things like logging/tracing, etc. Employing this monad gives your code the ability to produce this output on the side without getting in the way.

Key Pieces

• Functions in the Writer monad are decorated with Writer l r. l in this case is the type that you’ll be logging out where r is the result being returned from your function.
• The function tell is what’s used to push another value into the log/trace/writer.
• Operations in the Writer monad can be chained together using >>=
• runWriter is what you’ll use to run something in the Writer monad to get your result back.

An Example

import Control.Monad.Writer

-- | Starts a value off.
-- This function doesn't perform any calculation at all, it just prepares an
-- initial value to start in the calculation pipeline
--
start :: Int -> Writer [String] Int
start x = do
  tell (["Starting with " ++ show x])
  return x

-- | Halve a value
-- Any value passed into this function gets halved
--
half :: Int -> Writer [String] Int
half x = do
  tell (["Halving " ++ show x])
  return (x `div` 2)

-- | Squares a value
-- Any value passed into this function gets squared
--
sqr :: Int -> Writer [String] Int
sqr x = do
  tell (["Squaring " ++ show x])
  return (x * x)

main :: IO ()
main = do
  let work = runWriter $ start 10 >>= half >>= sqr >>= half
  let ans  = fst work
  let log  = snd work

  putStrLn $ "Answer: " ++ show ans
  putStrLn ""
  putStrLn " ==== Log ==== "

  mapM_ putStrLn log