Cannot use DefaultSignatures for TypeFamiles
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.
Trac metadata
Trac field | Value |
---|---|
Version | 8.6.2 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |