Hi,
There's nothing wrong with your type families. The problem is that the compiler
doesn't know that the m and rsc of eval are the same as m and rsc of runLoader.
(Also you had a small bug in the type of eval)
You need the ScopedTypeVariables extension, with a forall on runLoader to tell
GHC that they should be scoped:
runLoader :: forall m rsc a. (Monad m, Resource rsc) => CfgOf (IdOf rsc) ->
RscLoader rsc m a -> m a
runLoader cfg loader = viewT loader >>= eval M.empty
where
eval :: (Monad m, Resource rsc) =>
M.Map (IdOf rsc) rsc
-> ProgramViewT (EDSL (IdOf rsc)) m a
-> m a
eval _ (Return x) = return x
eval rscs (instr :>>= k) = case instr of
Load id -> do let loc = retrieveLoc cfg id
-- open and load from loc will go here
viewT (k ()) >>= eval rscs
-- -- -- Other cases yet to come...
greetings,
Sjoerd
On Nov 1, 2010, at 1:53 AM, Yves Parès wrote:
> Hello,
>
> I'm trying to make a simple monad (built on operational's ProgramT) for
> resource loading.
> I have classes featuring type families :
>
> {-# LANGUAGE TypeFamilies, FlexibleContexts, GADTs #-}
>
> -- | A ResourceId is something that identifies a resource.
> -- It should be unique for one resource, and should be used to find the
> location (the path) of the resource,
> -- possibly by using a configuration datatype
> class (Ord id) => ResourceId id where
> type LocOf id
> type CfgOf id
> retrieveLoc :: CfgOf id -> id -> LocOf id
>
> -- | Class describing a resource of type @rsc@
> class (ResourceId (IdOf rsc)) => Resource rsc where
> type IdOf rsc
> load :: LocOf (IdOf rsc) -> IO (Maybe rsc)
> -- ^ Called when a resource needs to be loaded
> unload :: rsc -> IO ()
> -- ^ Idem for unloading
>
> -- | Then, the operations that the loader can perform
> data EDSL id a where
> Load :: id -> EDSL id ()
> IsLoaded :: id -> EDSL id Bool
> Unload :: id -> EDSL id ()
>
> -- | The loader monad itself
> type RscLoader rsc m a = ProgramT (EDSL (IdOf rsc)) m a
>
> -- | And finally, how to run a loader
> runLoader :: (Monad m, Resource rsc) => CfgOf (IdOf rsc) -> RscLoader rsc m a
> -> m a
> runLoader cfg loader = viewT loader >>= eval M.empty
> where
> eval :: (Monad m, Resource rsc) =>
> M.Map (IdOf rsc) rsc
> -> ProgramViewT (EDSL rsc) m a
> -> m a
> eval _ (Return x) = return x
> eval rscs (instr :>>= k) = case instr of
> Load id -> do let loc = retrieveLoc cfg id
> -- open and load from loc will go here
> viewT (k ()) >>= eval rscs
> -- -- -- Other cases yet to come...
>
>
>
> Well, there is no way I can get it type-check. I think I must be misusing the
> type families (I tried with multi-param typeclasses and functional
> dependencies, but it ends up to be the same kind of nightmare...).
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
--
Sjoerd Visscher
sjo...@w3future.com
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