[Haskell-cafe] Deriving vs. type constructors

[Atze van der Ploeg]

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" <sperber at deinprogramm.de> 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
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