FizzBuzz.hs

import Control.Monad.Writer
import Control.Monad.Reader

type FizzBuzz = ReaderT Int (Writer String) ()

tellFizz :: FizzBuzz
tellFizz = do
  n <- ask
  when (n `rem` 3 == 0) $ tell "Fizz"

tellBuzz :: FizzBuzz
tellBuzz = do
  n <- ask
  when (n `rem` 5 == 0) $ tell "Buzz"

tellNumber :: FizzBuzz
tellNumber = do
  n <- ask
  when (n `rem` 3 /= 0 && n `rem` 5 /= 0) $ tell (show n)

fizzbuzz :: FizzBuzz
fizzbuzz = do
  tellFizz
  tellBuzz
  tellNumber

runFizzBuzz :: Int -> String
runFizzBuzz = execWriter . runReaderT fizzbuzz

main :: IO ()
main = mapM_ putStrLn $ map runFizzBuzz [1..30]


{-
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz
Fizz
22
23
Fizz
Buzz
26
Fizz
28
29
FizzBuzz
-}