On Sun, Feb 25, 2007 at 12:18:25AM +0300, Bulat Ziganshin wrote:

Hello Marc,

Friday, February 23, 2007, 5:22:12 PM, you wrote:

...

type ActionMonad a l = forall l. (HOccurs D1 l) => ( ReaderT l IO a )

'l' should be either parameter of type constructor or forall'ed variable. it seems that you try to set limitations on type constructor parameter - thing that has another syntax and anyway not much support in haskell'98. i suggest you to use smth like the following instead:

type ActionMonad a l = ( ReaderT l IO a )

instance (HOccurs D1 l) => Get CR (ActionMonad Bool ()) where get (CR a) = a

-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com Hi Bulat (and others) Thanks for your answer. I've tried doing this: a) The idea of using get instead of record accessing functions a1, a2 is overloading: ( let a = a1 record vs let (A a) <- get record let (A a) <- get record2 (record2 can be different type than record) ) benefit of the snd version: You can also see its type immideately. Because I've taught DrIft to derive Get a b .. its not much additional work.

b) The snd idea is using an invironment which is typesafe and easy to extend by using a HList and hOccurs to get the environment. I think this is great because you don't have to write getStateX, getEnvY = accessor . ask/get Im curious about reading you comments on a) b) ;) Happily Marc Weber ============= testfile - needs HList and GHC ========================= {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-incoherent-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} {-# OPTIONS -fno-monomorphism-restriction #-} module Main where import HList hiding ( liftIO ) import Control.Monad.Reader import Control.Monad.Trans import HOccurs class Get a b where get :: a -> b data D1 = D1 Int deriving (Show) -- dummy type data D2 = D2 Int deriving (Show) -- dummy type type ActionMonad l a = ReaderT l IO a newtype A l a = A (ActionMonad l a) newtype B l a = B (ActionMonad l a) data ActionRecord l a = ActionRecord { a1 :: A l a , a2 :: B l a } instance Get (ActionRecord l a) (A l a) where get ar = a1 ar instance Get (ActionRecord l a) (B l a) where get ar = a2 ar type RD1 = (HCons D1 HNil) type RD2D1 = (HCons D2 RD1) myActionRecord = ActionRecord (A act1) (B act2) where act1 :: ReaderT RD1 IO () act1 = do liftIO $ print "act 1" (d1@(D1 _)) <- asks hOccurs liftIO $ print (show d1) act2 :: ReaderT RD1 IO () act2 = do liftIO $ print "act 2" (d1@(D1 _)) <- asks hOccurs liftIO $ print (show d1) mytrans act3 $ (\l -> HCons (D2 7) l) -- adding new environment (D2 7) here -- addD2 act3 (D2 2) mytrans f tr = do a <- ask lift $ runReaderT f (tr a) -- here order doesn't matter: act3 :: ( HOccurs D1 (HCons a b) , HOccurs D2 (HCons a b)) => ReaderT (HCons a b) IO () act3 = do liftIO $ print "act 3 within act2" (d2@(D2 _)) <- asks hOccurs liftIO $ print (show d2) -- asks' :: (HOccurs l D2) => (l -> D2) -> ReaderT l IO D2 -- asks' = asks -- addD2 :: (HOccurs l' D2, HOccurs l D1) => ReaderT l' IO a -> D2 -> ReaderT l IO a -- addD2 m d2 = mytrans m (\l -> HCons d2 l) hcons2 :: a -> b -> HCons a (HCons b HNil) hcons2 a b = HCons a (HCons b HNil) main = let (A act1 :: A RD1 ()) = get myActionRecord (B act2 :: B RD1 ()) = get myActionRecord in do runReaderT (sequence [act1, act2]) (HCons (D1 1) HNil) runReaderT act3 (hcons2 (D2 7) (D1 1)) -- order doen't matter: runReaderT act3 (hcons2 (D1 3) (D2 7)) ============= testfile ===============================================