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Credit is due: I saw this originally on /r/haskell but the user deleted his or her comment so I'm unsure who the attribution belongs to.

Credit is due: I saw this originally on /r/haskell.

handleFizzBuzz :: String -> String
handleFizzBuzz =
    map $ unwords . fizzBuzz . convertInputLine

main :: IO ()
main = do
    args <- getArgs
    let path = args !! 0
    file <- readFile path
    putStrLn . unlines . handleFizzBuzz . lines $ file

fizzBuzzSingle :: Int -> Int -> Int -> Maybe String
fizzBuzzSingle f b n =
    ("F" <$ guard (mod n f == 0)) <> ("B" <$ guard (mod n b == 0))

fizzBuzz :: [Int] -> [String]
fizzBuzz [f, b, end] =
    map (fromMaybe . show <*> fizzBuzzSingle f b) [1 .. end]

main :: IO ()
main = do
    args <- getArgs
    file <- readFile (head args)
    (putStrLn . unlines . map (unwords . fizzBuzz . map read . words) . lines) file

handleFizzBuzz :: String -> String
handleFizzBuzz =
    map $ unwords . fizzBuzz . convertInputLine

main :: IO ()
main = do
    args <- getArgs
    let path = args !! 0
    file <- readFile path
    putStrLn . unlines . map handleFizzBuzz . lines $ file

import Data.Monoid
import Data.Maybe
import Control.Monad
import System.Environment

fizzBuzzSingle :: Int -> Int -> Int -> Maybe String
fizzBuzzSingle f b n =
    ("F" <$ guard (mod n f == 0)) <> ("B" <$ guard (mod n b == 0))

fizzBuzz :: [Int] -> [String]
fizzBuzz [f, b, end] =
    map (fromMaybe . show <*> fizzBuzzSingle f b) [1 .. end]

main :: IO ()
main = do
    args <- getArgs
    file <- readFile (head args)
    (putStrLn . unlines . map (unwords . fizzBuzz . map read . words) . lines) file

Some functions in the Prelude can be of help. One is map, which allows you to write a simpler handleFizzBuzz.

handleFizzBuzz :: String -> String
handleFizzBuzz =
    concatOutput . fizzBuzz . convertInputLine

main :: IO ()
main = do
    args <- getArgs
    let path = args !! 0
    file <- readFile path
    putStrLn . unlines . handleFizzBuzz . lines $ file

Another is unwords, which is equivalent to your concatOutput.

Some functions in the Prelude can be of help. One is map, which allows you to write a simpler handleFizzBuzz. Another is unwords, which is equivalent to your concatOutput.

handleFizzBuzz :: String -> String
handleFizzBuzz =
    map $ unwords . fizzBuzz . convertInputLine

main :: IO ()
main = do
    args <- getArgs
    let path = args !! 0
    file <- readFile path
    putStrLn . unlines . handleFizzBuzz . lines $ file