Hi Ralf, Thanks. I'm sorry, now I think that wasn't the source of my problem. What I want to do is specialise not to a specific type like Bool but to the class of all pairs (a,b). But this causes the compiler to complain, even for simpler examples: cast True :: (Typeable a, Typeable b) => Maybe (a,b) <interactive>:1:0: Ambiguous type variable `a' in the constraints: `Typeable a' arising from instantiating a type signature at <interactive>:1:0-51 `Show a' arising from use of `print' at Top level Probable fix: add a type signature that fixes these type variable(s) <interactive>:1:0: Ambiguous type variable `b' in the constraints: `Typeable b' arising from instantiating a type signature at <interactive>:1:0-51 `Show b' arising from use of `print' at Top level Probable fix: add a type signature that fixes these type variable(s) Is there a way to solve this, or do I have to avoid polymorphism? I can use 'toConstr' to find out dynamically if a particular type is a pair, and then use unsafeCoerce, but I hear that unsafeCoerce is unsafe. Frederik On Mon, Apr 03, 2006 at 05:41:55PM -0700, Ralf Lammel wrote:

...

Hi Ralf,

I'm looking for a function like extT but with more general type:

(t a -> s a) -> (t b -> s b) -> (t a -> s a)

Is there such a thing in the generics library?

Hi Frederik,

Not sure how you are exactly going to use such an operation ... But here is its implementation anyhow. Thanks for the riddle.

Ralf

import Data.Generics

-- Frederik's weird ext operation :-) ext' :: (Data (t a), Data (s a), Data (t b), Data (s b)) => (t a -> s a) -> (t b -> s b) -> (t a -> s a) ext' f g ta = case cast g of Just g' -> g' ta Nothing -> f ta

-- A generic default f (Just x) = [x] f Nothing = []

-- A type-specific case g (Just True) = [True] g (Just False) = [] g Nothing = []

-- A composition using our new type-extension operator test :: Data a => Maybe a -> [a] test = ext' f g

-- Let's see whether it works ... main = do print $ test (Just (1::Int)) print $ test (Just False)

-- http://ofb.net/~frederik/