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

[Steffen Schuldenzucker]

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 <sjoerd at w3future.com <mailto:sjoerd at 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...).
>     > _______________________________________________
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> 
>     --
>     Sjoerd Visscher
>     sjoerd at w3future.com <mailto:sjoerd at w3future.com>
> 
> 
> 
> 
> 
> 
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