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@gmail.com> wrote:

2009/7/21 Conal Elliott <conal@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.