Re: [Haskell-cafe] Loading a texture in OpenGL

It's possible ResourceT from the yesod guys could be quite useful. It allows you to manually release things, but also ensures that things are released in case of exceptions. It also supports, from what I can tell, some kind of reference counting for use with multiple threads (the deallocator isn't called unless all threads have released it, I think). Cheers, Alex On 8 February 2012 15:02, Clark Gaebel wrote:

Sounds hairy. Is there any way to get reference counting garbage collection in Haskell?

On Tue, Feb 7, 2012 at 9:26 AM, L Corbijn wrote:

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Just to clarify, this guarantee possibly could be made, ghc just doesn't do it now. In the past ghc never guaranteed a finalizer would ever be run.

Regardless I would be wary of trusting finalizers to clean up very scarce resources. A malloc'd buffer is probably fine to have a finalizer for, texture objects or file descriptors are a different matter. Predictability matters in those cases.

Sent from my iPhone^H^H^H^H^HPortable Turing machine

On Feb 6, 2012, at 10:16 PM, Austin Seipp wrote:

It's a precise GC of course (conservative collection would be madness considering how much memory Haskell programs chew through.) That still doesn't ensure your finalizer will run during the next GC even if all

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references are gone by then.

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On Feb 6, 2012, at 10:09 PM, Clark Gaebel wrote:

Is the Haskell garbage collector conservative, or precise?

If it's conservative, then this will only usually work. If it's

On Tue, Feb 7, 2012 at 5:23 AM, Austin Seipp wrote: the precise, it

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should always work.

On Mon, Feb 6, 2012 at 10:56 PM, Ben Lippmeier wrote:

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On 07/02/2012, at 2:50 PM, Clark Gaebel wrote:

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I would be running the GC manually at key points to make sure it gets cleaned up. Mainly, before any scene changes when basically

everything gets

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thrown out anyways.

From the docs:

newForeignPtr :: FinalizerPtr a -> Ptr a -> IO (ForeignPtr a)Source Turns a plain memory reference into a foreign pointer, and associates a finalizer with the reference. The finalizer will be executed after the last reference to the foreign object is dropped. There is no guarantee of promptness, however the finalizer will be executed before the program exits.

"No guarantee of promptness". Even if the GC knows your pointer is unreachable, it might choose not to call the finaliser. I think people have been bitten by this before.

Ben.

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You have to be really careful with automatic cleanup using finalizers. Not only to the mentioned not cleaning up of resources but also to deleting it too early. What could happen is that you code doesn't explicitly use (= call a GL function with it) an object after binding it. Then it could be seen as a dead pointer and be deleted by it's finalizer. But thereby it might unbind the object from the binding point. Maybe a (more) realistic example is using only one shader program, you create it once, call usePorgram with it and never change it after that. As there is no other program to use you don't have to reactivate the program with useProgram or have to change anything about it. So in effect it's not used by Haskell anymore and the finalizer is run for it deleting the program. For a program this is not a big problem as the OpenGL spec tells you that it isn't deleted immediately so you can use it afterwards.

With textures it's different, the are deleted immediately, so you may think you have a texture bound but in reality the finalizer might have run and deleted the texture for you. So watch doing the OpenGL memory management using the references in Haskell, as you might accidentally delete objects ahead of time.

Lars,

P.S. To make it worse, the bound objects (programs, textures, etc.) can also be queried and thereby there are non dead objects automatically, but there is no Haskell reference to them so the GC cannot now this.

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-- Alex Mason