Alexander Solla wrote:
Doing similar constructions with type classes is possible. I think you might have to use witness types (or even a nice functorial wrapper around your target value in the original algebra, or both) to do generalizations of type classes. For example:
class Uneditable obj where a :: a -> obj b :: b -> obj
class Refactored obj witness where a' :: Maybe (a -> obj) b' :: Maybe (a -> obj)
data EmptyFilter -- I think the name of the extension needed for this is 'EmptyDataDecls' data NoA data NoB
instance Uneditable obj => Refactored obj EmptyFilter where a' = Just a; b' = Just b instance Uneditable obj => Refactored obj NoA where a' = Nothing; b' = Just b Alexander, Thank you for your excellent explanation and example. Due to my lack of knowledge, I do not fully understand what is going in the example.
I had a look at the types of a and a’. *Main> :t a a :: forall a obj. (Uneditable obj) => a -> obj *Main> :t a' a' :: forall witness a obj. (Refactored obj witness) => Maybe (a -> obj) Could you explain the example a little more. Apologies for my slowness. Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie