Data.Ratio and exceptions

[Kazu Yamamoto ( 山本和彦 )]

Hello,

> The essential nature of the function wouldn't be altered any, it'd
> just give a more precise error. Seems like an improvement regardless
> of how one feels about errors/exceptions/bottoms.
> 
> Also, I think using DivideByZero is appropriate here. No need to
> invent a new exception type.

GHC.Real uses the error function in some places. I believe that most
cases should be throw ArithException. I will survey and make a
comprehensive proposal.

If there are other modules to be surveyed, please let me know.

--Kazu