[Haskell-cafe] Question on proper use of Data.IORef
Hi experts, I fear I don't understand how to properly use *Data.IORef*. I wrote the following code: 1 -- Testing Data.IORef 2 module Main where 3 4 import Data.IORef 5 6 bump :: IORef Int - IO() 7 bump theRef = do 8 tmp - readIORef theRef 9 let tmp2 = tmp + 1 10 writeIORef theRef tmp2 11 12 main = do 13 let theValue = 1 14 print theValue 15 theValueRef - newIORef theValue 16 bump theValueRef 17 return theValue and got this, in ghci: *Main :load test2.hs [1 of 1] Compiling Main ( test2.hs, interpreted ) Ok, modules loaded: Main. *Main main 1 *1* I was expecting this: *Main :load test2.hs [1 of 1] Compiling Main ( test2.hs, interpreted ) Ok, modules loaded: Main. *Main main 1 *2* Can anyone help me understand what I'm doing wrong? Thanks! -db ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question on proper use of Data.IORef
theValueRef isn't a pointer to theValue that you can use to somehow change theValue (which is immutable). theValueRef is a reference to a box that contains a totally separate, mutable value. When you use newIORef to create theValueRef, it's *copying* theValue into the box. When you mutate theValueRef, you're mutating the value inside the box - theValue remains unchanged. Cheers, Adam On 22 June 2012 11:30, Captain Freako capn.fre...@gmail.com wrote: Hi experts, I fear I don't understand how to properly use *Data.IORef*. I wrote the following code: 1 -- Testing Data.IORef 2 module Main where 3 4 import Data.IORef 5 6 bump :: IORef Int - IO() 7 bump theRef = do 8 tmp - readIORef theRef 9 let tmp2 = tmp + 1 10 writeIORef theRef tmp2 11 12 main = do 13 let theValue = 1 14 print theValue 15 theValueRef - newIORef theValue 16 bump theValueRef 17 return theValue and got this, in ghci: *Main :load test2.hs [1 of 1] Compiling Main ( test2.hs, interpreted ) Ok, modules loaded: Main. *Main main 1 *1* I was expecting this: *Main :load test2.hs [1 of 1] Compiling Main ( test2.hs, interpreted ) Ok, modules loaded: Main. *Main main 1 *2* Can anyone help me understand what I'm doing wrong? Thanks! -db ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question on proper use of Data.IORef
On Fri, Jun 22, 2012 at 5:30 PM, Captain Freako capn.fre...@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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question on proper use of Data.IORef
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 initialize itself. 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 in Haskell. 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 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?* Thanks, all! -db On Fri, Jun 22, 2012 at 8:43 AM, Max Rabkin max.rab...@gmail.com wrote: On Fri, Jun 22, 2012 at 5:30 PM, Captain Freako capn.fre...@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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question on proper use of Data.IORef
On Fri, Jun 22, 2012 at 11:18 AM, Captain Freako capn.fre...@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.rab...@gmail.com wrote: On Fri, Jun 22, 2012 at 5:30 PM, Captain Freako capn.fre...@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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
