[squeak-dev] Re: immutibility

Bert Freudenberg bert at freudenbergs.de
Wed Mar 31 13:07:25 UTC 2010

On 31.03.2010, at 15:03, Igor Stasenko wrote:
> On 30 March 2010 14:09, Bert Freudenberg <bert at freudenbergs.de> wrote:
>> On 30.03.2010, at 11:45, Bryce Kampjes wrote:
>>> On Thu, 2010-03-18 at 16:40 +0100, Bert Freudenberg wrote:
>>>> On 18.03.2010, at 16:30, Ralph Johnson wrote:
>>>>> On 3/18/10, Bert Freudenberg <bert at freudenbergs.de> wrote:
>>>>>> With true immutables you can start developing in a mixed object/functional style, which would allow interesting optimizations, e.g. for concurrency, memoization etc.
>>>>> I've developed in a mixed object/functional style for years, and I
>>>>> think a lot of other people do, too.   You don't need language support
>>>>> for this, though I expect it would be helpful.  It makes concurrency
>>>>> and distributed programming easier, and is important when you are
>>>>> dealing with databases.  Eric Evans calls this "Value Object" in his
>>>>> book "Domain Driven Design".
>>>>> -Ralph Johnson
>>>> Right, the style is possible without VM support in your own subsystem. But being able to *guarantee* immutability would allow to generalize this beyond your own special-purpose code. And possibly it would enable further VM-level optimizations.
>>> Even perfectly VM enforced immutability isn't enough to allow
>>> optimisation if you still allow become:. It's always possible to swap
>>> out the immutable object with a different one via become:.
>> It's fine to swap immutable objects, unless the object holding the reference is itself unmutable. No instance variable of an immutable object can be stored into, not even by become:.  Obviously, become: would have to honor the immutability flag:. For references inside immutable objects, become: becomes a forward-become (or a no-op if both were immutable):
>> a := Array with: A new.
>> b := Array with: B new beImmutable.
>> c := (Array with: C new) beImmutable.
>> a first become: b first. "a points to the immutable B instance, b to the A instance".
>> a first become: c first. "both a and c now point to the immutable C instance"
> one of the problem with #become:/becomeForward: that it should swap
> all of existing references atomically.
> Now imagine that while walking the heap, it replaced a 50% of
> references to given object, and then discoreved that it
> needs to swap a reference held by immutable object.
> Now, what you suppose to do? Revert all changes and fail primitive?
> Ignore this object (do not alter it references) and continue?
> Or, before doing the real swapping, scan the heap and check all object
> to make sure that none of them are immutable and so, #become will work
> ok?

Just ignore all writes into immutable objects (that is, in the become code, do not insert a forwarding block), otherwise proceed as usual.

- Bert -

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