Parsec.Char：oneOf

everyday parser Parsec 
Prelude> :m Text.ParserCombinators.Parsec
Prelude Text.ParserCombinators.Parsec> :t oneOf
oneOf :: forall st. [Char] -> CharParser st Char

module Main where

import Text.ParserCombinators.Parsec

vowel = oneOf "aeiou"

run :: Show a => Parser a -> String -> IO ()
run p input
        = case (parse p "" input) of
            Left err -> do{ putStr "parse error at "
                          ; print err
                          }
            Right x  -> print x

で、

Prelude> :l 20080930_parsec00
Compiling Main             ( 20080930_parsec00.hs, interpreted )
Ok, modules loaded: Main.
*Main> run vowel "a"
Loading package parsec ... linking ... done.
'a'
*Main> run vowel "iueo"
'i'
*Main> run vowel "k"
parse error at (line 1, column 1):
unexpected "k"