repl.hs

{-# LANGUAGE GHC2021 #-}
module Main where

import Data.Char ( isDigit )
import Data.Maybe ( mapMaybe )

main :: IO ()
main = interact (drive repl)

drive :: ([String] -> [String]) -> (String -> String)
drive f = unlines . f . lines

repl :: [String] -> [String]
repl ins = mapMaybe output $ eval $ initState ins

data MState = MState { inputs :: [String], output :: Maybe String }

initState :: [String] -> MState
initState ins = MState { inputs = ins, output = Nothing }

eval :: MState -> [MState]
eval state = state : rests
    where
        rests | isFinal state = []
              | otherwise     = eval (step state)

isFinal :: MState -> Bool
isFinal state = case inputs state of
    "bye":_ -> True
    _       -> False

step :: MState -> MState
step state = case inputs state of
    l:ls -> if all isDigit l
        then state { inputs = ls, output = Just (fizzbuzz (read l)) }
        else state { inputs = ls, output = Nothing }

fizzbuzz :: Int -> String
fizzbuzz n
    | n `mod` 15 == 0 = "FizzBuzz"
    | n `mod`  5 == 0 = "Buzz"
    | n `mod`  3 == 0 = "Fizz"
    | otherwise       = show n