[Haskell-cafe] How odd...

[Ian Lynagh]

On Sat, Aug 04, 2007 at 04:38:08PM +0100, Andrew Coppin wrote:
> 
> BTW, I recently had some code like this:
> 
>  foo x
>    | x < 0 = ...
>    | x == 0 = ...
>    | x > 0 = ...
> 
> I was most perplexed when I got a "non-exhaustive patterns" exception... 
> It turns out there was a NaN in there. I forget about that.

If you use guards then you can get non-exhaustive pattern warnings
anyway, e.g.:

    myNot x
     | x == True  = False
     | x == False = True

gives

    Warning: Pattern match(es) are non-exhaustive
             In the definition of `myNot': Patterns not matched: _

as it is (in general) undecidable whether all patterns are covered.

> >While working through this I also came across the following case which 
> >technically is a bug:
> >
> >> 0 ** 0
> >1.0
> >
> >> exp (log 0 * 0)
> >NaN
> >
> >I suspect that GHCi is using a built-in exponentiation operator that 
> >doesn't quite conform to the standard in this case.

Hmm, according to the report the Floating class has a default method
    x ** y           =  exp (log x * y)
but the Double instance is just
    instance  Floating   Double  where ...
so could define something different.

6.4.3 says
    0**y is undefined.


Thanks
Ian