[Haskell-cafe] Resource module

[Bas van Dijk]

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)

Regards,

Bas


On Wed, Jun 2, 2010 at 2:28 PM, Bas van Dijk <v.dijk.bas at gmail.com> 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)
>
> Regards,
>
> Bas
>
> On Wed, Jun 2, 2010 at 1:11 AM, Arie Peterson <ariep at xs4all.nl> wrote:
>> On Tue, 1 Jun 2010 21:10:40 +0200, Bas van Dijk <v.dijk.bas at gmail.com>
>> wrote:
>> | [...]
>> | Hi Arie, I would love to see some examples of these resources for
>> | which you can't define a Resource[1] instance.
>> | [...]
>> |
>> | [1]
>> |
>> http://hackage.haskell.org/packages/archive/regions/0.5/doc/html/Control-Resource.html
>>
>>
>> I had this involved example of a function that takes a resource, and
>> returns a similar resource, which performs the relevant IO actions in a
>> separate thread, receiving its instructions over a concurrent channel.
>> However, in the course of explaining why it doesn't fit in the simple
>> open/Handle/close framework, I actually helped myself to see that it is
>> possible (and not difficult) :-).
>>
>>
>> A different scenario where the open/Handle/close framework may actually
>> not suffice is the following:
>>
>>> fallback :: Resource cap IO -> Resource cap IO -> Resource cap IO
>>> fallback (Resource primary) (Resource backup) = Resource l where
>>>   l c = primary c `catch` (\ ProblemWithMainResource -> backup c)
>>
>> ; the fact that @c@, the "continuation" (which describes how the
>> capability is used), is mentioned twice in the body of @l@ makes this a
>> weird case.
>>
>>
>> By the way, Bas, I'm not quite sure how to properly use your Resource
>> class. Should one create different datatypes for different resources, if
>> they have different handle types or open/close functions, even though they
>> provide the same "capability"? I would like to avoid this, if possible, to
>> make life easier for users of these resources (they just want a resource
>> providing a certain capability, and don't care about its internal state). I
>> suppose one can create a class of resources giving a certain capability
>> instead.
>>
>>
>> Kind regards,
>>
>> Arie
>>
>>
>