Parsing Ratio with zero denominator

[Henning Thielemann]

On Sun, 31 May 2020, Dannyu NDos wrote:

> 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)


I think you are right. A readPrec parser should only generate parser 
errors and should be total. I do not know if anyone relies on the current 
behavior. I suspect that most users are not aware of the problem and their 
programs will be aborted in case of zero denominator where they shouldn't.