> Instances.hs:1:
> Duplicate or overlapping instance declarations
> for `HasConfig (arcTypeConfig value) (arcTypeParms value)'
> defined at Instances.hs:19 and defined at Instances.hs:11
>
> This is not fair, because while the instances for HasConfig have the
> potential to conflict in the future, they will only do so should the classes
> NodeTypeConfigParms and ArcTypeConfigParms overlap.
It is fair. Let's rename your type variables to make what's going on a little clearer:
main = return ()
class HasConfig a b where
($$) :: a -> b -> b
class NodeTypeConfigParms a b where
nodeTypeConfig :: a c -> b c -> b c
instance (NodeTypeConfigParms a b) => HasConfig (a c) (b c) where
($$) = nodeTypeConfig
class ArcTypeConfigParms a b where
arcTypeConfig :: a c -> b c -> b c
instance (ArcTypeConfigParms a b) => HasConfig (a c) (b c) where
($$) = arcTypeConfig
You can now see that the two instance declarations overlap: their
right hand sides are in fact *identical*. Remember that the
typechecker simply matches on the right-hand sides ("heads") of the
instance declarations.
If you do -fallow-undecidable-instances, I think your program will
work (in the Prolog-ish backtracking way), but note that if
NodeTypeConfigParms and ArcTypeConfigParms are ever given instances at
the same pair of types, the value of ($$) will be undefined.
--KW 8-)
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