Monoid FizzBuzz

gistfile1.hs

import Data.Monoid

data FizzBuzz = N | I Int | S String

instance Monoid FizzBuzz where
    mempty              = N

    N    `mappend` N    = N
    N    `mappend` I i  = S $ show i
    N    `mappend` S x  = S x
    I i  `mappend` N    = S $ show i
    I _  `mappend` S x  = S x
    S x  `mappend` I _  = S x
    S x  `mappend` S y  = S $ x <> y
    S x  `mappend` N    = S x
    I _  `mappend` I _  = error "FizzBuzz"

fizzbuzz :: [String]
fizzbuzz = map (\(S x) -> x) fbs
  where
    fizz = cycle [N, N, S "Fizz"]
    buzz = cycle [N, N, N, N, S "Buzz"]
    is = map I [1..]
    fbs = zipWith (<>) is $ zipWith (<>) fizz buzz