[Haskell-cafe] Deriving vs. type constructors

[Erik Hesselink]

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
<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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