[Haskell-cafe] Deriving vs. type constructors

[Atze van der Ploeg]

Possibly you can work around this issue uainh the constraints package
http://hackage.haskell.org/package/constraints
On Oct 15, 2014 6:18 PM, "Atze van der Ploeg" <atzeus at gmail.com> wrote:

> 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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> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
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