yet another very simple overlapping instance example

[Razvan Musaloiu-E.]

Hi!

Keith Wansbrough wrote:
>>instance Op_plus MyInt MyInt where
>>instance (Num a) => Op_plus a MyInt where
>>instance (Num a) => Op_plus MyInt a where
> 
> [..]
> 
>>Overlapping instance declarations:
>>   multi.hs:9: Op_plus a MyInt
>>   multi.hs:12: Op_plus MyInt a
>>Failed, modules loaded: none.
> 
> The GHC manual talks about this at:
> 
> http://haskell.cs.yale.edu/ghc/docs/latest/html/users_guide/type-extensions.html#INSTANCE-DECLS
> 
> I think the issue is that GHC still requires that overlapping
> instances either do not unify, or have an instantiation ordering.
> Your 2nd and 3rd instances are unordered: 2 is an instance of 3, and 3
> is an instance of 2.  GHC doesn't notice the first instance.

As you mention that documentation, it says:

	GHC is also conservative about committing to an overlapping
	instance. For example:

	  class C a where { op :: a -> a }
	  instance C [Int] where ...
	  instance C a => C [a] where ...

	  f :: C b => [b] -> [b]
	  f x = op x

	From the RHS of f we get the constraint C [b]. But GHC does not
	commit to the second instance declaration, because in a
	paricular call of f, b might be instantiate to Int, so the first
	instance declaration would be appropriate. So GHC rejects the
	program. If you add -fallow-incoherent-instances GHC will
	                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	instead silently pick the second instance, without complaining
	about the problem of subsequent instantiations.

I tried -fallow-incoherent-instances but it only worked on that example 
even the two examples seems to be the same problem. Am I right?

Obs: I'm working with a GHC version 5.04.2. Does it make sens to try the 
latest 6.0 one?

> I observe that the docs above say "Yell if this restriction bites
> you.", so I shall hand this over to the GHC developers to discuss
> further...

Big 10x! :-) Should I further explain why do I need something like this? 
And if yes, where should I do this?

--
Razvan ME