我试图更好地了解readPlibrary：
为什么会这样：

pLookIf :: Parser String 
pLookIf = (do
        looked <- look --munch isDigit
        e1 <- munch isDigit
        if looked == e1 then return ( "(" ++ e1 ++ ")" )
        else return "88"
        return looked
        )

parseString input = readP_to_S (do                         
                        e <- pLookIf
                        eof
                        return e) input

返回：

ghci> parseString "8888888888888"
[("8888888888888","")]

当这个

pLook :: Parser String 
pLook = (do
        looked <- look
        return looked
        )

parseString input = readP_to_S (do                         
                        e <- pLook
                        eof
                        return e) input

退货：

ghci> parseString "8888888888888"
[]

在这两种情况下，都应该返回一些值，考虑签名：look :: ReadP String