On 16-Dec-2003, John Meacham <[EMAIL PROTECTED]> wrote: > newtype Id a = Id a > > type Er f = f (E f) -- E used recursivly > data E f = EAp (Er f) (Er f) | ELam Int (Er f) | > ELetRec [(Int,Er f)] (Er f) | EVar Int > > [...] problem 2 persists. If I don't want to be constantly > casting to and from Id, I have to use both the seperate annotated and > unannotated versions. > > is this actually the case? is there a better solution I don't see? perhaps > something using crazy GHC extensions? if not, are there any proposed > extensions which would solve this promlem?
I think views <http://www.haskell.org/development/views.html> would solve this problem, wouldn't they? You could define a view of `E Id' with constructors that skip over the conversions to/from Id view EId of E Id = App (E Id) (E Id) | Lam Int (E Id) | LetRec [(Int,E Id)] (E Id) | Var Int where eid (EAp (Id x) (Id y)) = Ap x y eid (ELam x (Id y)) = Lam x y eid (ELetRec (B bindings) (Id e)) = LetRec bindings' e where bindings' = map (\(v,(Id x))->(v,x)) bindings eid (EVar v) = Var v Then you can traverse expressions without needing to write any explicit conversions to/from Id, e.g. -- "occurs v e" returns True iff v occurs somewhere in e occurs :: Int -> (E Id) -> Bool occurs v (Lam v1 e) = v == v1 || occurs v e occurs v (Ap x y) = occurs v x || occurs v y occurs v (LetRec bindings e) = any (\(vi,ei)->v==vi || occurs v ei) bindings || occurs v e occurs v (Var v1) = v == v1 -- Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit The University of Melbourne | of excellence is a lethal habit" WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp. _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
