[Haskell-cafe] Question on proper use of Data.IORef
capn.freako at gmail.com
Fri Jun 22 18:18:44 CEST 2012
Okay, I get it now. Thanks to all of you for your quick replies!
So, here's what I need to do:
1. My Haskell code gets called by a higher level C function and asked to
2. As part of that initialization, I'm expected to return a pointer to
an instance of a particular data structure, which gets created/initialized
3. I create a stable pointer to a data structure instance, using *
newStablePtr*, and return that pointer to the calling C function.
4. 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
*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?*
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>
> > 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.
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