Re: [Haskell-cafe] Question on proper use of Data.IORef

On Fri, Jun 22, 2012 at 11:18 AM, Captain Freako wrote:

Okay, I get it now. Thanks to all of you for your quick replies!

So, here's what I need to do:

My Haskell code gets called by a higher level C function and asked to initialize itself. As part of that initialization, I'm expected to return a pointer to an instance of a particular data structure, which gets created/initialized in Haskell. I create a stable pointer to a data structure instance, using newStablePtr, and return that pointer to the calling C function. The same C function then, at a later time, calls a different function in my Haskell code, sending it that very same pointer, and expects that code to modify the data structure pointed to, in place.

So, I wrote some code that looks like this:

secondFunction :: StablePtr DataStructure -> IO () secondFunction dsPtr = do     ds <- deRefStablePtr dsPtr     theRef <- newIORef ds     writeIORef theRef newDs

which, of course, didn't work, because I didn't understand the true nature of IORef.

So, my question is: How do I do this? That is, how do I modify, in place, a data structure, which I originally created and made stable, using a pointer to that structure, which is being passed back to me, by the higher level C function that is calling me?

You could use a StablePtr (IORef DataStructure).

Thanks, all! -db

On Fri, Jun 22, 2012 at 8:43 AM, Max Rabkin wrote:

...

On Fri, Jun 22, 2012 at 5:30 PM, Captain Freako wrote:

...

 12 main = do  13     let theValue = 1  14     print theValue  15     theValueRef <- newIORef theValue  16     bump theValueRef  17     return theValue

theValue is a plain old immutable Haskell variable. "newIORef" creates an IORef whose initial value is equal to the argument; it doesn't create a pointer to the value or something like that. Change "return theValue" to "readIORef theValueRef" to extract the changed value in the IORef.

--Max

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe