This isn't a GHC limitation. The report specifies that the class
hierarchy must be a DAG. So C cannot require itself as a prerequisite,
even if it's on a 'different' type.
Practically, in the implementation strategy that GHC (and doubtless
other compilers) use, the declaration:
class C (A x) => C x ...
means that a C x dictionary contains a C (A x) dictionary, which
contains a C (A (A x)) dictionary.... And dictionaries are strictly
evaluated, so this sort of infinite definition cannot work.
-- Dan
On Wed, Jul 20, 2011 at 12:37 PM, Ryan Trinkle
The following code doesn't compile, but it seems sensible enough to me. Is this a limitation of GHC or is there something I'm missing?
class C (A x) => C x where type A x :: *
instance C Int where type A Int = String
instance C String where type A String = Int
The error I get is:
SuperclassCycle.hs:1:1: Cycle in class declarations (via superclasses): SuperclassCycle.hs:(1,1)-(2,15): class C (A x) => C x where { type family A x :: *; }
Ryan
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users