Norbert, On Wed, Jan 25, 2017 at 3:36 PM, Norbert Hartl <norb...@hartl.name> wrote:
> Clément, > > Am 25.01.2017 um 13:54 schrieb Clément Bera <bera.clem...@gmail.com>: > > > > On Wed, Jan 25, 2017 at 11:35 AM, Norbert Hartl <norb...@hartl.name> wro > te: > >> Does anyone know the state of immutability support in vm and image? The >> latest vm downloadable is compiled with >> >> IMMUTABILITY=1 >> >> (Esteban said that). When I open a pharo6 image with this VM and do: >> >> ASUser new >> setIsReadOnlyObject: true; >> name: 'foo' >> >> with >> >> ASUser>>#name: arg1 >> name := arg1 >> >> I don't get an exception. Is there something missing or am I not >> understanding? >> > > Hi Norbert, > > Thank you very much for looking read-only objects. > > When mutating an instance variable, the VM triggers a call-back that by > default does nothing. In your case, running your code does not raise an > exception but the object should not be modified either. If you want an > exception, you need to change the call-back code, i.e., the method > Object>>#attemptToAssign: value withIndex: index. For example, you could > write: > > Object>>#attemptToAssign: value withIndex: index > | process | > self notify: 'object changed !'. > process := Processor activeProcess. > [ process suspendedContext: process suspendedContext sender ] forkAt: > Processor activePriority + 1. > Processor yield. > > Then, your code should open a notification window with 'object changed', > and proceeding keeps running the code without mutating the object. > > thank you very much for the explanation. What was the rationale behind > doing nothing as default? I can see there is two interpretations of > read-only. One being just don't modify the object the other being throwing > an exception when an attempt to modify is made. I think that having an > exception thrown would make it easier to write code using it. I don't want > to monkey patch Object but still make this general applicable. > Well, when I introduced the code I thought someone would build a ModificationTracker framework... I think there should be a modification tracker framework that throws an exception only if no program is registered to handle the modification failure. No exception should be thrown in the generic case. > > One needs to build a ModificationTracker framework on top of the VM > support I introduced. Multiple things are required, like default behavior > in this call-back and in primitive failure code. I am willing to support > and help anyone willing to build such a framework, but I won't build it > myself. > > Modification tracking is exactly the reason why I look into it. I have two > other approaches doing modification tracking. But both are inferior to an > approach using read-only objects. > Overall, you need to: - change the code of all numbered primitives mutating objects (such as at:put:) so that when they fail because of a read-only object they call the modification tracker framework. - add the call to the modification tracker framework from the #attemptToAssign:withIndex: call-back. - build a Modification tracker framework where external programs can easily register themselves to catch modification on specific objects or all instances of a specific class. Programs tracking modifications should be able to temporarily make the object writable, perform the modification and make it read-only again. You can check the VisualWork implementation (check for example the fall-back code of #at:put: and what it calls) and the comment I wrote in #attemptToAssign:withIndex: to get inspired. If you do something like that, please, *please*, please contribute it back to the base Pharo image. Thank you for experimenting with read-only objects. Don't hesitate to contact me if you have issues or questions. > Norbert > > If you have any other questions or if you find bug don't hesitate to ask > further questions > > Best, > > PS: Make sure "Smalltalk vm supportsWriteBarrier" answers true in your > system, if this is not the case it means the VM does not support read-only > objects. > > Clement > > > > > > >> >> Norbert > > >
