[Haskell-cafe] GADT and instance deriving

[Alexander Solla]

On Fri, May 24, 2013 at 9:37 AM, TP <paratribulations at free.fr> wrote:

> Hello,
>
> I continue my learning of "not so obvious" Haskell/GHC topics when
> encountering problems in the code I write.
> Below is a small example of an heterogeneous list, using GADT, inspired
> from:
>
> http://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types#Example:_heterogeneous_lists
>
> ----------
> {-# LANGUAGE GADTs #-}
>
> data Box where
>     Box :: Eq s => s -> Box
>
> instance Eq Box where
>
>     (Box s1) == (Box s2) = s1 == s2
> ----------
>
> This code does not compile, because GHC is not sure that s1 and s2 have the
> same type:
>
> ----------
> Could not deduce (s ~ s1)
>     from the context (Eq s)
>       bound by a pattern with constructor
>                  Box :: forall s. Eq s => s -> Box,
>                in an equation for `=='
>       at test_eq_GADT_before.hs:8:6-11
> [and more lines...]
> ----------
>
> (Do you confirm that tilde in s~s1 means "s has the same type as s1"? I
> have
> not found this information explicitly in the Haskell stuff I have read).
>

Yes.

http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/equality-constraints.html

Is this (Typeable) the right way to go? Is there any other solution?
>

Using typeable is a perfectly reasonable way to go.
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