Just out of curiosity: Does it work if you omit eval's type signature?
In fact you can't omit it since EDSL is a GADT.
I don't know why there is this restriction, but it is written in
operational's documentation:
http://hackage.haskell.org/packages/archive/operational/0.2.0.1/doc/html/Con...
(At the very bottom of the page)
But there still must be something I don't get :
I tried to merge the two classes ResourceId and Resource in only one
Resource class, which leads to a few changes in runLoader :
{-# LANGUAGE TypeFamilies, FlexibleContexts, GADTs, ScopedTypeVariables #-}
import qualified Data.Map as M
import Control.Monad.Operational
-- | 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
-- | Class describing a resource of type @rsc@
class (Ord (IdOf rsc)) => Resource rsc where
type IdOf rsc
type LocOf rsc
type CfgOf rsc
retrieveLoc :: CfgOf rsc -> IdOf rsc -> LocOf rsc
load :: LocOf 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 :: forall m rsc a. (Monad m, Resource rsc)
=> CfgOf 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...
This leads to new errors with 'IdOf rsc' and 'CfgOf rsc' :
GameBasics/Resources.hs:46:42:
Couldn't match expected type `CfgOf rsc'
against inferred type `CfgOf rsc1'
NB: `CfgOf' is a type function, and may not be injective
In the first argument of `retrieveLoc', namely `cfg'
In the expression: retrieveLoc cfg id
In the definition of `loc': loc = retrieveLoc cfg id
GameBasics/Resources.hs:46:46:
Couldn't match expected type `IdOf rsc'
against inferred type `IdOf rsc1'
NB: `IdOf' is a type function, and may not be injective
In the second argument of `retrieveLoc', namely `id'
In the expression: retrieveLoc cfg id
In the definition of `loc': loc = retrieveLoc cfg id
Seems like the compiler still has a 'rsc1' type despite the scoped type
variable 'rsc'.
2010/11/1 Yves Parès
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 !
2010/11/1 Sjoerd Visscher
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 sjoerd@w3future.com