Sebastian Fischer wrote:
Heinrich Apfelmus wrote:
[...] you can implement your type as
newtype CMaybe a = CMaybe { forall b . (a -> [b]) -> [b] }
Yes. For me it was interesting to see how far we get by wrapping `Maybe` in `Codensity`: we get more than `Maybe` but not as much as `[]`.
Well, you can implement it with Maybe as well, at the price of duplicated computations. The idea is that for the implementation of orElse , we're not interested in the full list of results, only in whether this list is empty or not. This leads to eval :: ProgramView Language a -> Maybe a eval (Return a :>>= k) = Just a eval (MZero :>>= k) = Nothing eval (MPlus n m :>>= k) = (eval . view) (n >>= k) `mplus` (eval . view) (m >>= k) eval (OrElse n m :>>= k) = case (eval . view) n of Nothing -> (eval . view) (m >>= k) Just _ -> (eval . view) (n >>= k) Thanks to lazy evaluation, this is not too inefficient, even. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com