Polymorphic lists...
MR K P SCHUPKE
k.schupke at imperial.ac.uk
Tue Mar 9 14:41:47 EST 2004
I have written a first attempt at a fold function for the heterogenious list:
class RFold i r where
rFold :: (forall a . a -> i -> i) -> i -> r -> i
instance RFold i RNil where
rFold f i RNil = i
instance RFold i r => RFold i (a `RCons` r) where
rFold f i (x `RCons` xs) = f x (rFold f i xs)
This works providing the folded 'op' has the type: forall a . a -> i -> i
which means it does not work for functions like show :: forall a . Show a => a -> i -> i
as the types are different. I have not figured out a way to make it accept a constraint
like Show for example. Here is an example:
length = rFold (\_ -> (+1)) 0 relation
The use of such a function seems limited, if constraints like Show cannot be used, as
most useful applications of fold would require some kind of class membership for example:
string = rFold shows "" relation
This fails to compile because shows has type:
shows :: forall a . Show a => a -> i -> i
but fold expects
op :: forall a . a -> i -> i
Regards,
Keean Schupke.
More information about the Glasgow-haskell-users
mailing list