overlapping instances and functional dependencies

[Wolfgang Jeltsch]

Hello,

I think, I realized now what my mistake was. The handling of overlapping 
instances comes into play when the compiler has to decide which method 
definition to choose for a specific instance. It is not for choosing one of 
more possible instances.

In my example, C Int (Int,Char,Bool) Int and C Int (Int,Char,Bool) Char where 
both candidates for instances of C. Because they match the pattern of the 
first instance declaration (which is the more general one), both candidates 
actually have to be instances. The handling of overlapping instances only 
states that the compiler has to use the method definitions of the second 
instance declaration for the instance C (Int,Char,Bool) Char.

Am I correct?

Wolfgang


On Monday, 2003-08-18, 00:37, CEST, Wolfgang Jeltsch wrote:
> I wrote on Saturday, 2003-08-09, 01:32, CEST:
> > Hello,
> >
> > I have this code:
> >     class C a b c | a b -> c where
> >         f :: a -> b -> c
> >
> >     instance C a b c => C a (x,y,b) c where
> >         f a (_,_,b) = f a b
> >
> >     instance C a (a,c,b) c where
> >         f _ (_,c,_) = c
> > ghci -fglasgow-exts -fallow-overlapping-instances compiles it without
> > complaint but hugs -98 +o says:
> >     ERROR "ClassProblem.hs":7 - Instances are not consistent with
> >     dependencies
> >     *** This instance    : C a (a,b,c) b
> >     *** Conflicts with   : C a (b,c,d) e
> >     *** For class        : C a b c
> >     *** Under dependency : a b -> c
> > Can anyone tell me what the reason for this is and, maybe, how to avoid
> > these problems with Hugs?
> >
> > Wolfgang
>
> Hal Daume answered on Saturday, 2003-08-09, 01:53, CEST:
> > Suppose somewhere we have an instance:
> >
> >  instance C Int Bool Int
> >
> > when the first instance decl you have says we also have
> >
> >   instance C Int (x,y,Bool) Int
> >
> > in this case, Int + (x,y,Bool) should uniq. specify Int.
> >
> > however, we also have:
> >
> >   instance C a (a,c,b) c
> >
> > where, if we let a=Int, b=Bool, c=Char, then we get that
> >   Int + (Int,Char,Bool) should uniq. specify Char.
> >
> > these two confict because if, in the first case, we instantiate x to Int
> > and y to Char, then one says that the third param should be a Bool and
> > the other says the third param should be a Char.
> >
> > (or at least this is my understanding -- someone might correct me)
> >
> >  - Hal
>
> Hello,
>
> I wouldn't suppose that there is a conflict in your example. The question
> is for which t there is an instance C Int (Int,Char,Bool) t. There are two
> competing instance declarations:
>     (1) instance C a b c => C a (x,y,b) c [...]
>         Because in your example there is an instance C Int Bool Int we
> would get the instance C Int (Int,Char,Bool) Int.
>     (2) instance C a (a,c,b) c [...]
>         This clearly votes for C Int (Int,Char,Bool) Char.
> But the second instance declaration is more specific. In the first one we
> have the "pattern" "arbitrary type - triple type - arbitrary type" with no
> further restrictions. In the second one we have the same "pattern" but with
> the restriction that the first parameter must equal the type of the first
> triple element and the third parameter must equal the type of the second
> triple element. Because of the handling of overlapping instances, the
> compiler should take the second instance declaration and deduce the
> instance C Int (Int,Char,Bool) Char.
>
> What's wrong with my interpretation?
>
> Wolfgang