Hi Chris,
With the following type, and transformation functions:
data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show)
t1,t2,t3 :: Even -> Maybe Even
But if one of the transformations has a different type, you can't do it this way. For instance, redefine t2 to have a different type:
t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing
and you are stuck because the functions of different types can't be combined into a single transformation.
My question is: is there a good way to combine the transformation functions if they have different types?
Currently, no. Although there is something definitely related, with transformBis: http://hackage.haskell.org/packages/archive/uniplate/1.6.10/doc/html/Data-Ge... That takes a list of transformation functions of different types and acts as though you did transform on each one in turn. You could certainly imagine adding rewriteBis in the same style, and with your version you almost have. The transformBis function is particularly efficient because it "knows" which traversals or parts of traversals can be fused without changing the semantics. rewriteBis could certainly do the same trick. If you provided a patch for rewriteBis I'd certainly apply it! Thanks, Neil