[Haskell-cafe] Resource module

[Bas van Dijk]

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
>
>