[Haskell-cafe] Re: Simple GADT parser for the eval example

...No surprise there, since there is no way to fail in the event of a maltyped "Expr". The next thing to try is a type class solution...

Think about it: you need to do type checking. For our little language we used Template Haskell to do your "Expr -> Term a" translation, thus the type-checking of our input programs is actually done by Haskell's type checker ;-) See our PLPV paper http://www.iro.umontreal.ca/~monnier/tct.pdf Stefan