Re: [Haskell-cafe] Resource module

2 Jun 2010

      On Wed, Jun 2, 2010 at 2:57 PM, Arie Peterson  wrote:
...
...
On Wed, Jun 2, 2010 at 2:28 PM, Bas van Dijk 
wrote:
...
Before answering your questions I would like to make sure I understand
your Resource type. When I want to create a memory Resource for
example is the following what you have in mind?
{-# LANGUAGE Rank2Types #-}
-- from base:
import Foreign.Ptr ( Ptr )
import Foreign.Marshal.Alloc ( mallocBytes, free )
-- from transformers:
import Control.Monad.IO.Class ( liftIO )
-- from MonadCatchIO-transformers:
import Control.Monad.CatchIO ( MonadCatchIO, bracket )
newtype Resource cap m = Resource { with :: forall a. (cap -> m a) -> m
a }
type Memory m a = Resource (Ptr a) m
memory :: MonadCatchIO m => Int -> Memory m a
memory size = Resource $ bracket (liftIO $ mallocBytes size) (liftIO .
free)
Yes, exactly. I also create type aliases for resources providing a
specific capability.
On Wed, 2 Jun 2010 14:45:08 +0200, Bas van Dijk 
wrote:
...
The previous can also be generalized using my Resource class:
-- from regions:
import qualified Control.Resource as C ( Resource(..) )
resource :: (MonadCatchIO m, C.Resource resource)
         => resource -> Resource (C.Handle resource) m
resource r = Resource $ bracket (liftIO $ C.open r) (liftIO . C.close)
Yes, definitely.
(This is not a literal generalisation of the 'memory' function, unless you
make 'Int' an instance of 'C.Resource'; one would probably create a special
type 'data Memory = Memory Int' instead. This is the difference I alluded
to in my earlier email.)
Indeed.

My regional-pointers package provides this Memory type and an instance
for Resource:
http://hackage.haskell.org/packages/archive/regional-pointers/0.1.0.2/doc/ht...

Now I'm beginning to understand the problem with your 'fallback'
function. If I specialize the capability to Handles to Memory as in:

-- from regional-pointers:
import Foreign.Ptr.Region ( Memory )

fallback :: Resource (C.Handle (Memory a)) IO
         -> Resource (C.Handle (Memory a)) IO
         -> Resource (C.Handle (Memory a)) IO
fallback (Resource primary) (Resource backup) = Resource l
    where
      l c = primary c `catch` (\(SomeException _) -> backup c)

then the only way to create a value of type 'Resource (C.Handle
(Memory a))' is to apply my previously posted 'resource' function to
'Memory nrOfBytes'. There's no other way to create Handles to Memory.
Is that your problem?

The problem is that Handle is an associated _data_ type not an
associated _type synonym_:

class Resource resource where
    data Handle resource ∷ *
    open ∷ resource → IO (Handle resource)
    close ∷ Handle resource → IO ()

So 'Handle' is an injective type function which means that every
'Handle resource' has only one 'resource'.

I could try turning 'Handle' into an associated type synonym.

Regards,

Bas