FizzBuzz.hs

{-# LANGUAGE ExistentialQuantification #-}
import Data.Foldable
import Data.Set (Set)
import qualified Data.Set as Set
import Prelude hiding (mapM_)

data FizzBuzzTag = Fizz | Buzz deriving (Eq, Ord, Show)

type FizzBuzz = Set FizzBuzzTag

showSetAscending :: Show a => Set a -> String
showSetAscending s = foldMap show (Set.toAscList s)

fizz :: Integral a => a -> FizzBuzz
fizz n = case n `rem` 3 of
          0 -> Set.singleton Fizz
          _ -> Set.empty

buzz :: Integral a => a -> FizzBuzz
buzz n = case n `rem` 5 of
          0 -> Set.singleton Buzz
          _ -> Set.empty

fizzbuzz :: Integral a => a -> FizzBuzz
fizzbuzz n = fizz n `Set.union` buzz n

data FB = forall a. (Show a, Integral a) => FB (Either a FizzBuzz)

fb :: (Show a, Integral a) => a -> FB
fb n = FB $ let tags = fizzbuzz n in
  if Set.null tags then Left n else Right tags

instance Show FB where
  show (FB x) = either show showSetAscending x

main = mapM_ (putStrLn . show . fb) [1..50]

I think I've spotted a bug. If you make FizzBuzz a set, you lose ordering, so you might get 15 => "BuzzFizz", no?

@krisajenkins the desire to get the correct ordering is exactly why I call the toAscList fn.