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
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