The append example is meant to make a general point about the connection between dependent types and Haskell with extensions. I know that the example itself is rather trivial, and DML/index types, Omega, Haskell+GAD and Chameleon might seem as too big canons for a rather small target.
Second you can express append in a much simpler and readable way in Haskell using type classes:
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data Nil deriving Show data List a = Cons a (List a) deriving Show
class Append l l' l'' | l l' -> l'' where append :: l -> l' -> l'' instance List l' => Append Nil l' l' where append _ l' = l' instance Append l l' l'' => Append (Cons a l) l' (Cons a l'') where append (Cons a l) l' = Cons a (append l l')
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Thats the complete definition, no need for extra stuff or complex exqualities or casts...
Doesn't look too bad to me.
Good, I was waiting for some type class trickery :) You turn clauses into instance declarations. This will only work for terminating programs! Martin