GADTs? data CompletePathEv p where CompletePathEv :: CompletePath p => CompletePathEv p class Path p => IncompletePath p where type CompletedPath p :: * completedPathEv :: CompletePathEv (CompletedPath p) Later you can pattern-match on completedPathEv and get your CompletePath instance back. On 1 Jul 2010, at 22:09, Andrew Coppin wrote:
Consider the following:
class Path p where ...
class Path p => CompletePath p where ...
class Path p => IncompletePath p where type CompletedPath p :: *
Obviously, the idea is that CompletedPath Foo (where Foo is an IncompletePath) should yield some type which is a CompletePath. However, the source code does not actually state this, so GHC (rightly) complains that it was unable to deduce this constraint from the actual code written.
Is it possible to state this constraint? If so, how? (And if not, the answer, presumably, is to fundamentally redesign the whole thing...)
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