n+k patterns

[Malcolm Wallace]

> I argued that (Num a, Ord a) makes most sense to me.
> You argued that (Integral a) was a conscious choice (something I
> don't remember but I'm sure you're right), and is the right one anyway.
> 
> I'd be interested to know what others think.  If there's any doubt,
> we'll stay with Integral.

My view is that (n+k) patterns are evil, so it doesn't really matter
what we decide.  :-)  No, seriously, I'm a little worried about
widening the range of numeric types for which (n+k) patterns are
supposed to work.  I can (just about) imagine wanting to use Rationals
in an (n+k) pattern, but Float and Double?  I'm not convinced that
would be useful.

    f :: Double -> Double
    f (n+1) = n

    Main> print (f 3.0)
    2.0
    Main> print (f 2.0001)
    1.0001000000000002
    Main> print (f 1.0000000001)
    1.000000082740371e-10

Regards,
    Malcolm