[Haskell] deriving Show for GADT?

[Edward Kmett]

I seem to recall Matt Morrow having some code lying around for automatically
generating such instances using haskell-src-exts. I wonder how hard it would
be to adapt to Template Haskell.
-Edward Kmett


On Tue, Apr 14, 2009 at 2:03 AM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> You might be able to write some Template Haskell to derive the Show
> instance.
>
> It's a bit tricky, because there are some types which can't have Show
> derived, such as:
>
> data Foo where
>    Broken :: a -> Foo
>
> What should
>    show (Broken id)
> do?
>
>  -- ryan
>
> On Mon, Apr 13, 2009 at 9:28 PM, Norman Ramsey <nr at cs.tufts.edu> wrote:
> > I've got a fairly large GADT for which I wished to use
> >   deriving (Show)
> > but I got a mysterious error message:
> >
> > Exp.hs:13:11:
> >    Can't make a derived instance of `Show (Exp a)'
> >      (`Exp' has non-Haskell-98 constructor(s))
> >    In the data type declaration for `Exp'
> >
> >
> > This is from GHC.  Does anybody know a compiler option or other trick
> > that will coax the compiler into producing a Show instance.
> > (I know I can write one by hand, but I'd rather not bother.)
> >
> >
> > Norman
> > _______________________________________________
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> >
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