#16027: Cannot use DefaultSignatures for TypeFamiles -------------------------------------+------------------------------------- Reporter: Ericson2314 | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'm surprised there wasn't a ticket for this. Maybe I just couldn't find it. In short, I want to do something like: {{{ class Monad m => MyMonad m where type AssociatedMonad m :: Type -> Type default type AssociatedMonad m :: (m ~ t m', MonadTrans t, MyMonad m') => Type -> Type type AssociatedMonad (t m') = t (AssociatedMonad m') fun :: Foo -> m () default fun :: (m ~ t m', MonadTrans t, MyMonad m') => Foo -> m () fun = lift . fun }}} The syntax is a bit weird, but I hope in the process of implementing constrained type families the syntax properly analogous to the term-level would shake itself out. The use-case, like the normal method original, is to assist with monad transformer boilerplate. {{{MonadTrans}}} wouldn't be enough, but {{{MFunctor}}} from mmorph might be depending on the method using the associated type. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16027 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler