[HOpenGL] Pure, garbage-collected graphics resources

[Conal Elliott]

Thanks for these comments


> Anyway, I've not a clear picture of what you have in mind (especially,
> at which point in time a, say, VBO should be considered to be part of
> things to be rendered). Often, a data structure (say Blah) is created
> in a pure way then given to some kind of run :: Blah -> IO () function
> for "interpretation". Why not mirror the OpenGL API in a purely
> data-centric way then give the data to the run function ?
>

This "data-centric" style (I like that term) is exactly what I like to do in
all of my libraries, and it's what I'm doing again now.  OpenGL types are
hidden in the library implementation, and the exposed semantics is
unaffected by the imperativeness of OpenGL (just as the semantics of Integer
is unaffected by the imperativeness of 'print').

And then I find myself in an implementation puzzle.  My 'run' function (for
the purely functional type) involves creating VBOs and textures, which
greatly accelerate rendering.  I want to *reuse* these resources without
mutation (i.e. reuse the content, not the memory), which is easy if they're
wrapped up as functional values that go into my higher-level functional
values, but tricky if they get created only during 'run'.  Another example
is shader programs, which I synthesize and then never mutate.

Since I'm using this graphics data in a purely functional way, I want both
creation and destruction to be handled in a functional way, i.e., lazy
evaluation and garbage collection, with no visible IO.  I think we can
finally get there, now that we have dependable prompt execution of C-based
finalizers.

However, I do not think robust finalization can be added on top of the Ptr
type, which is used in HOpenGL, because GHC optimizations often drop
constructors (like Ptr) keeping only the contents, which allows finalizers
to get called much too soon.  Some examples of this general problem are
mentioned in the addFinalizer documentation [1].  I think a solution would
be to replace Ptr with ForeignPtr in HOpenGL.

   - Conal

[1]
http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html#v%3AaddFinalizer


On Tue, Jul 21, 2009 at 4:30 AM, minh thu <noteed at gmail.com> wrote:

> 2009/7/21 Conal Elliott <conal at conal.net>:
> > I'd like to use some OpenGL resources (VBOs, textures, shaders, and
> shader
> > programs) in a functional way, with immutability, garbage collection, and
> > IO-free creation interfaces.  Has anyone played with doing such a thing?
>  I
> > guess the GC part would involve foreign pointers with foreign finalizers
> > (which now run promptly in GHC iiuc).  I don't know of any reliable way
> to
> > add finalizers to Ptr values, because of the unboxing problem [1].
> >
> > One tricky issue is that graphics context initialization must take place
> > before any of these "pure" resources get evaluated.  If the APIs allowed
> > access to to multiple graphics contexts, things would get stickier.
> >
> > Comments?
>
> Hi,
>
> I wanted to point you to a paper (Stretching the storage manager: weak
> pointers and stable names in Haskell) but see you're one of the
> authors.
>
> As you say, there is the notion of context. I guess you can create the
> context with something explicitely in IO, like
>
> createContext :: IO Context
>
> then implementing the "pure" resources as data structure referencing
> the context.
>
> Anyway, I've not a clear picture of what you have in mind (especially,
> at which point in time a, say, VBO should be considered to be part of
> things to be rendered). Often, a data structure (say Blah) is created
> in a pure way then given to some kind of run :: Blah -> IO () function
> for "interpretation". Why not mirror the OpenGL API in a purely
> data-centric way then give the data to the run function ?
>
> Although the OpenGL C API is imperative, it maps fairly well to a
> data-centric approach.
>
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