[Haskell-cafe] GADT and instance deriving

[TP]

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).

I have googled on the topic, and found:
http://stackoverflow.com/questions/6028424/defining-eq-instance-for-haskell-gadts