Hi Greg, Since hfoldr is right-associative, I prefer to reorder your list of functions as follows:
test = HCons (length::String -> Int) (HCons ((+1)::(Int->Int)) (HCons ((*2)::(Int->Int)) HNil))
Note that I also annotated length with its specific type. (If you really wanted to leave things more polymorphic, you would need to engage in TypeCast.) Providing a specific Apply instance for (.) is not necessary, strictly necessary. We could try to exploit the normal function instance for Apply. Let me recall that one here for convenience:
instance Apply (x -> y) x y where apply f x = f x
Let me also recall the hFoldr instances:
class HList l => HFoldr f v l r | f v l -> r where hFoldr :: f -> v -> l -> r
instance HFoldr f v HNil v where hFoldr _ v _ = v
instance ( HFoldr f v l r , Apply f (e,r) r' ) => HFoldr f v (HCons e l) r' where hFoldr f v (HCons e l) = apply f (e,hFoldr f v l)
To fit in (.), we would flip and uncurry it. So we could try: comp' = hFoldr (uncurry (flip (.))) (id::Int -> Int) test This wouldn't work. The trouble is the required polymorphism of the first argument of hFoldr. The type of that argument as such is polymorphic. However, this polymorphism does not survive type class parameterization. You see this by looking at the HCons instance of HFoldr. The different occurrences of "f" would need to be used at different types. This would only work if the type class parameter f were instantiated by the polymorphic type of (uncurry (flip (.))). (And even then we may need something like TypeCast.) What you can do is define a dedicated *type code* for composition. comp = hFoldr (undefined::Comp) (id::Int -> Int) test data Comp instance Apply Comp (x -> y,y -> z) (x -> z) where apply _ (f,g) = g . f Ralf
-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- bounces@haskell.org] On Behalf Of Greg Buchholz Sent: Sunday, November 06, 2005 7:01 PM To: haskell-cafe@haskell.org Subject: [Haskell-cafe] Type classes and hFoldr from HList
I was playing around with the HList library from the paper...
Strongly typed heterogeneous collections http://homepages.cwi.nl/~ralf/HList/
...and I thought I'd try to fold the composition function (.) through a heterogeneous list of functions, using hFoldr...
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-}
import CommonMain
main = print $ comp "abc"
test = HCons ((+1)::(Int->Int)) (HCons ((*2)::(Int->Int)) (HCons length HNil))
comp = hFoldr (.) id test
instance Apply (a -> b -> c -> d) (a, b) (c -> d) where apply f (a,b) = f a b
...but it fails with the following type error...
]Compiling Main ( compose.hs, interpreted ) ] ]compose.hs:10:7: ] No instances for (Apply ((b -> c) -> (a -> b) -> a -> c) ] (Int -> Int, r) ] ([Char] -> a3), ] Apply ((b -> c) -> (a -> b) -> a -> c) (Int -> Int, r1) r, ] Apply ((b -> c) -> (a -> b) -> a -> c) ([a2] -> Int, a1 ->a1) r1) ] arising from use of `hFoldr' at compose.hs:10:7-12 ] Probable fix: ] add an instance declaration for (Apply ((b -> c) -> (a -> b) -> a -
c) ] (Int -> Int, r) ] ([Char] -> a3), ] Apply ((b -> c) -> (a -> b) -> a - c) ](Int -> Int, r1) r, ] Apply ((b -> c) -> (a -> b) -> a - c) ]([a2] -> Int, a1 -> a1) r1) ] In the definition of `comp': comp = hFoldr (.) id test
...Anyway, I couldn't quite tell whether I was using hFoldr incorrectly, or if I needed to have more constraints placed on the construction of "test", or if needed some sort of type-level function that resolves...
Apply ((b -> c) -> (a -> b) -> a -> c)
...into (a -> c), or something else altogether. I figured someone might be able to help point me in the right direction.