Re: [Haskell-cafe] Am I using type families well?

Hi Yves, On 11/01/2010 09:44 PM, Yves Parès wrote:

Yes, I did make a small mistake in the type of eval. In fact, through the compiler messages, I guessed that it was a problem of matching between the 'rsc' type variable of runLoader and the 'rsc' of eval. I thought that this kind of matching was automatic in Haskell, well I was wrong... Thanks !

Just out of curiosity: Does it work if you omit eval's type signature? -- Steffen

2010/11/1 Sjoerd Visscher mailto:sjoerd@w3future.com>

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 mailto:Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Sjoerd Visscher sjoerd@w3future.com mailto:sjoerd@w3future.com

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