Neil Mitchell wrote: > data ConsT a > data NilT > > data List a t where > Cons :: a -> List a b -> List a (ConsT b) > Nil :: List a NilT
Stefan O'Rear wrote: > data VarList a = forall t. VarList (List a t) > > fromListV :: [a] -> VarList a > fromListV [] = VarList Nil > fromListV (x:xs) = case fromListV xs of > VarList xs' -> VarList (Cons x xs') > > fromListC :: [a] -> (forall t. List a t -> r) -> r > fromListC [] fn = fn Nil > fromListC (x:xs) fn = fromListC xs (\sl -> fn (Cons x sl)) I noticed that fromListV and fromListC always force traversal of the input list. I made various attempts to modify them to preserve laziness, but this always resulted in a type error. For example: > fromListV (x:xs) = case fromListV xs of > ~(VarList l) -> VarList (Cons x l) Couldn't match the rigid variable `a' against the rigid variable `a1' `a' is bound by the type signature for `fromListV' `a1' is bound by the pattern for `VarList' at gadt-list.hs:33:42-50 Expected type: List a b Inferred type: List a1 t In the second argument of `Cons', namely `l' In the first argument of `VarList', namely `(Cons x l)' I guess the strict traversal of the input list is inevitable, considering that the concrete type of any (List a t) depends on the length of the list (even when hidden behind an existential). _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
