[Haskell-cafe] Overlapping Instances with Functional Dependencies

[Martin Sulzmann]

Hi,

I wouldn't call this a bug, overlapping instances 
and in particular the combination with functional dependencies
are something which is not well studied yet.
Hence, GHC is very conservative here.

I feel like you, this program should work.
As you correctly point out, there's a conflict among the
two improvement rules (resulting from the instances and FD).
A sensible decision is to apply the same "ad-hoc"
mechanism to improvement rules that is currently
applied to overlapping instances. Of course, we need some
formal system to express such conditions precisely.
You find some hints how to achieve this in

G. J. Duck, S. Peyton-Jones, P. J. Stuckey, and M. Sulzmann. 
Sound and decidable type inference for functional dependencies. 
In Proc. of ESOP'04

Martin


Daniel Brown writes:
 > I have the following three programs:
 > 
 >    class Foo a b
 >    instance Foo (a -> b) (a -> [b])
 >    instance Foo a a
 > 
 >    class Bar a b | a -> b
 >    instance Bar (a -> b) (a -> b)
 >    instance Bar a a
 > 
 >    class Baz a b | a -> b
 >    instance Baz (a -> b) (a -> [b])
 >    instance Baz a a
 > 
 > When compiled in ghc 6.4 (with -fglasgow-exts
 > -fallow-overlapping-instances -fallow-undecidable-instances) Foo
 > and Bar compile fine, but Baz fails with this error:
 > 
 >    Baz.hs:2:0:
 >        Functional dependencies conflict between instance declarations:
 >          Baz.hs:2:0: instance Baz (a -> b) (a -> [b])
 >          Baz.hs:3:0: instance Baz a a
 > 
 > This is how I interpret the error: The fundep says "a uniquely 
 > determines b", but if you have `Baz (Int -> Int) b`, b is `Int -> [Int]` 
 > according to the first instance and `Int -> Int` according to the second 
 > instance. b isn't uniquely determined by a, so the functional dependency 
 > isn't functional -- thus the conflict.
 > 
 > When confronted with overlapping instances, the compiler chooses the 
 > most specific one (if it is unique), e.g. `Baz (a -> b) (a -> [b])` is 
 > more specific than `Baz a a`.
 > 
 > But it seems that the combination of the two features is broken: if the 
 > most specific instance is chosen before checking the functional 
 > dependency, then the fundep is satisfied; if the fundep is checked 
 > before choosing the most specific instance, then it isn't.
 > 
 > Is this a bug, or am I confused?
 > 
 >   Dan
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