Philippa Cowderoy wrote:
> For example, GADTs let you implement monads as interpreters by defining a
> datatype representing the abstract syntax tree that describes a
> computation - you can't get this to type without at a minimum existential
> types and for many monad operations you need the full power of GADTs to
> declare a corresponding constructor.
I'm yet to see the example of that need. I have seen the examples that
the need for GADT was _claimed_ -- but then it turns out the example
is implementable without GADT after all. Here are a few such
examples: implementing State monad in a free term algebra
Initial (term) algebra for a state monad
http://www.haskell.org/pipermail/haskell-cafe/2005-January/008241.html
Implementing an interpreter in HOAS with fix
Even higher-order abstract syntax: typeclasses vs GADT
http://www.haskell.org/pipermail/haskell/2007-January/019012.html
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