Parsec.Combinator：choice

everyday parser Parsec 
module Main where

import Text.ParserCombinators.Parsec

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、

*Main> :l 20081004_parsec00.hs
Compiling Main             ( 20081004_parsec00.hs, interpreted )
Ok, modules loaded: Main.
*Main> run (choice [digit, space]) "0"
'0'
*Main> run (choice [digit, space]) "a"
parse error at (line 1, column 1):
unexpected "a"
expecting digit or space
*Main> run (choice [digit, space]) " "
' '