I mean, shouldn't there be a guard?

instance (Integral a, Read a) => ReadPrec (Ratio a) where
    readPrec = parens . prec 7 $ do
        n <- step readPrec
        lift (expect (Symbol "%"))
        d <- step readPrec
        guard (0 /= d)
        return (n % d)

2020년 5월 31일 (일) 오후 12:13, Dannyu NDos <ndospark320@gmail.com>님이 작성:

Shouldn't the parser be pfail rather than throwing an error?