Re: [wxhaskell-users] Some WX questions - a couple of missing bindings and a segfault

[Jules B. <ju...@je...>]

Eric Y. Kow wrote:
> Here's my implementation of it.  Does it look sane to you?  Any comments
> from wxhaskell developers?
> 
> withImageData :: Image a -> (Ptr () -> IO b) -> IO b
> withImageData image f = do
>     pixels <- imageGetData image
>     x <- f pixels
>     x `seq` image `seq` return x
> 
> The seqs are there out of pure superstition (i.e. Eric doesn't know how
> Haskell works); basically, I want to make sure we're holding on to image
> as long as we need to, so at least until we know what 'x' is.

The x `seq` is likely pointless. "x" is a pure value and unless 
someone's cheating with unsafePerformIO, merely forcing a pure value 
can't cause any Ptr access to happen.

the image `seq` looks like a reasonable hack to keep image alive.

The canonical way would be "touchForeignPtr image" or to use 
"withForeignPtr image", except that image isn't a ForeignPtr exactly. I 
don't know if there is a good way to access the ForeignPtr inside image 
because I haven't grokked enough of wx yet.

See 
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign-ForeignPtr.html 
for these and related issues.

> 
> If this is indeed sane, we could stick it in wxcore and put a big
> warning on the Haddocks for imageGetData (without removing it just
> yet).

And fix imageGetPixels to use the new safe withImageData. Which would 
have made my bug go away.

Jules