[Haskell] generalised algebraic data types, existential types, and phantom types

[Abraham Egnor]

I briefly skimmed the paper mentioned in a recent mailing on the
ghc-users list that describes generalised algebraic data types
(http://research.microsoft.com/Users/simonpj/papers/gadt/index.htm);
my reaction can be summed up as "nifty!".

I was curious to see if I could implement anything similar in current
haskell, and ended up with the attached code.  The main point of
interest for me is the combination of phantom types and existential
types; I'd never had the need to combine the two before.

Is there any closer approximation possible?  My code is close, but the
type-safety isn't checked by the compiler (although it should be safe,
if only the smart constructors are used), and it has that extra
"Typeable" constraint.

Abe
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