Counts vowels in a string and returns the count. Written in Haskell

vowels.hs

import Data.Set(member, fromList)
import System.Environment(getArgs)

setVowel = fromList "aeiou"

countVowels :: String -> [Integer]
countVowels word = fmap oneLetter word

oneLetter :: Char -> Integer
oneLetter letter = if member letter setVowel then 1 else 0

getVowelCount = sum . concat . (fmap countVowels)

main = do
     phrase <- getArgs
     putStrLn ("There are " ++ (show (getVowelCount phrase)) ++ " vowels.")