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

[Antoine Latter]

On Fri, Jun 22, 2012 at 11:18 AM, Captain Freako <capn.freako at gmail.com> 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 <max.rabkin at gmail.com> wrote:
>
>>
>> On Fri, Jun 22, 2012 at 5:30 PM, Captain Freako <capn.freako at gmail.com>
>> 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
>>
>
>
>
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