On Sun, Jul 25, 2010 at 1:53 PM, Alexey Karakulov ankaraku...@gmail.com wrote:
Suppose I have one piece of code like this:
class Result r e | r - e where
failure :: e - r a
success :: a - r a
Maybe instance is discarding failure information:
instance Result Maybe e where
Suppose I have one piece of code like this:
class Result r e | r - e where
failure :: e - r a
success :: a - r a
at :: Result r String = [a] - Int - r a
at xs i = if i = 0 i length xs
then success (xs !! i)
else failure Wrong index
Either instance of
But what to do with Maybe?
instance Result Maybe where
type Failure Maybe = forall e. e -- can't do this
failure _ = Nothing
success x = Just x
Normally, I would use: type Failure Maybe = ()
Unless the ability to discard information of any type is somehow a salient
feature.
