I don't think this currently possible. I think you are asking for the following GHC ticket: https://ghc.haskell.org/trac/ghc/ticket/5927 On Oct 15, 2014 4:56 PM, "Michael Sperber" wrote:

I'm struggling with a problem related to generic programming, boiling down to this:

----snip---- {-# LANGUAGE StandaloneDeriving, KindSignatures, UndecidableInstances #-}

data Foo (v :: * -> *) = Foo (v Bar)

deriving instance (Show (v Bar)) => Show (Foo v)

data Bar = Bar deriving Show ----snip----

So the Show instance for Foo v wants the rather specific Show instance for v Bar.

But now imagine that v is always something like Maybe, itself done using something like this presumably:

instance Show a => Show (Maybe a) where ...

Can I somehow state as a constraint that Foo v has a Show instance if v is a type constructor like Maybe, i.e. once with a Show instances for any arguments that has a Show instance?

(You can't see *why* I want to do this in this example - I actually have something like 100 nested datatypes which thread v through, and *every single one of them* adds to the context I need for every one of them.)

I feel I want to write something like this:

deriving instance (Show a => Show (v a)) => Show (Foo v)

Is there a way to do this?

Help would be much appreciated!

-- Regards, Mike _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

It's hard to know if this will solve your problem, but sometimes it helps to make your type more generic, and recover the original type using a type synonym. Something like this: {-# LANGUAGE StandaloneDeriving, KindSignatures, UndecidableInstances #-} data FooG a (v :: * -> *) = Foo (v a) type Foo = FooG Bar deriving instance (Show (v a)) => Show (FooG a v) data Bar = Bar deriving Show You don't even need the standalone deriving now, but it shows that the constraint will now be less specific. Another alternative could be to use something like `Show` from `Data.Functor.Classes` in transformers. That would mean you'd have to write your own `Show` instance, though. Erik On Wed, Oct 15, 2014 at 4:55 PM, Michael Sperber wrote:

I'm struggling with a problem related to generic programming, boiling down to this:

----snip---- {-# LANGUAGE StandaloneDeriving, KindSignatures, UndecidableInstances #-}

data Foo (v :: * -> *) = Foo (v Bar)

deriving instance (Show (v Bar)) => Show (Foo v)

data Bar = Bar deriving Show ----snip----

So the Show instance for Foo v wants the rather specific Show instance for v Bar.

But now imagine that v is always something like Maybe, itself done using something like this presumably:

instance Show a => Show (Maybe a) where ...

Can I somehow state as a constraint that Foo v has a Show instance if v is a type constructor like Maybe, i.e. once with a Show instances for any arguments that has a Show instance?

(You can't see *why* I want to do this in this example - I actually have something like 100 nested datatypes which thread v through, and *every single one of them* adds to the context I need for every one of them.)

I feel I want to write something like this:

deriving instance (Show a => Show (v a)) => Show (Foo v)

Is there a way to do this?

Help would be much appreciated!

-- Regards, Mike _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe