Singular Type Constructor

[Ashley Yakeley]

At 2001-08-12 11:26, Levent Erkok wrote:

>How about using overlapping instances? Add:
>
>  instance Eq a where
>     _ == _ = True
>
>to your program: if there isn't a ``better'' instance, it'll use this one. 
>Such an instance declaration is inherently dangerous to have around, but
>might help your particular situation.

How do Hugs and GHC calculate "better" instances in this case?

I make a distinction between "appears to overlap" and "actually 
overlaps": the first is what the compilers detect: I only switch off this 
checking if I have instances that "appear to" overlap but don't 
"actually" overlap e.g.:

--
instance MyClass Bool
instance (Num a) => MyClass a
--

...which don't "actually" overlap provided I don't have "instance Num 
Bool" anywhere.

Patching GHC so that it detects instances that "actually" overlap instead 
of "appear to" overlap is left as an exercise for the reader...

-- 
Ashley Yakeley, Seattle WA