Yes, indeed. Thanks again, (and thanks for building all the arrow notation infrastructure in the first place - it's awesome!) --Ben On 15 Jan 2012, at 11:57, Ross Paterson wrote:
On Sun, Jan 15, 2012 at 11:42:28AM +0000, Ben Moseley wrote:
The real application is trying to process a structure containing GADTs - something more like this:
{-# LANGUAGE GADTs,Arrows,Rank2Types #-} import Control.Arrow
data G a where G1 :: Int -> G Char G2 :: Int -> G Bool
-- mapcomb :: Arrow a => (a (env,G x) (G x)) -> a (env,(G b,G c)) (G b,G c) -- mapcomb _aegg = proc (_env,bc) -> returnA -< bc
process :: Int -> G x -> G x process i (G1 n) = G1 $ succ n process i (G2 n) = G2 $ succ n
processA :: Arrow a => a (Int,G x) (G x) processA = proc (i,gx) -> returnA -< process i gx
mapcomb :: Arrow a => (forall x . a (env,G x) (G x)) -> a (env,(G b,G c)) (G b,G c) mapcomb aegg = proc (env,(g1,g2)) -> do g1' <- aegg -< (env,g1) g2' <- aegg -< (env,g2) returnA -< (g1',g2')
myarr :: Arrow a => a Int Bool myarr = proc i -> do (|mapcomb (\g -> processA -< (i,g)) |) (G1 3,G2 3) -- (| (mapcomb (processA <<^ (\(_,g)->(7,g)) )) |) (G1 3,G2 3) returnA -< False
I guess to do this it'll be necessary to plumb 'i' through manually (changing the type of 'mapcomb') ?
In this particular case you could give mapcomb two arguments:
mapcomb :: Arrow a => a (env,G b) (G b) -> a (env,G c) (G c) -> a (env,(G b,G c)) (G b,G c)
and pass processA twice, but that wouldn't work in general (with an unlimited number of variants).
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