[Haskell-cafe] Associated types
andrewcoppin at btinternet.com
Thu Jul 1 14:09:36 EDT 2010
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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