[Vm-dev] Re: [Pharo-dev] Should literals be immutable ?

[Nicolas Cellier]

2014-10-28 0:19 GMT+01:00 Levente Uzonyi <leves at elte.hu>:

>
> On Mon, 27 Oct 2014, stepharo wrote:
>
>  Levente
>>
>> I think that the first step is not to make sure that you cannot hack
>> inside literals like mad. But to make sure that in 99.99 % of the case,
>> this assumption can hold.
>>
>> Stef
>>
>
> Let me sum it up:
> - The question is (by Clement) if it's okay to not implement the slow path.
> - He assumes that he doesn't have to implement it if literals are
> immutable.
> - I showed that literal immutability is not enough to avoid impelemting
> the slow path.
>
> So immutability doesn't affect anything about the need for implementing of
> the slow path. If Clement decides to not implement it, then one will be
> able to get the system into an inconsistent state, no matter if literals
> are immutable or not.
>
> If you think that the inconsistent state can only be achieved by hacking,
> then here's another example without "hacks":
>
> SomeClass >> #someMethod
>
>         ^someBoolean
>                 ifTrue: [ Array with: 1 with: 2 with: 3 ]
>                 ifFalse: [ Array empty "we don't have to create a new
> array for this, right? :)" ]
>
> SomeOtherClass >> #someOtherMethod: anArray
>
>         | copy |
>         anArray doSomething.
>         "let's grow that array"
>         copy := Array new: anArray size.
>         copy replaceFrom: 1 to: anArray size with: anArray startingAt: 1.
>         anArray becomeForward: copy. "this is really cheap in Spur (and in
> VW; even collections and streams use it)"
>
> ThirdClass >> #yetAnotherMethod: aSomeClass
>
>         | x |
>         x := aSomeClass someMethod.
>         y someOtherMethod: x.
>         ^x
>
> And now, when #yetAnotherMethod: is sent, we swap the literal in Array
> class >> #empty with another non-empty array. Based on Eliot's comment
> about VW, I'm pretty sure the VW guys had problems like this, despite of
> having immutable literals.
>
> Levente
>

But is there a becomeForward: in VW?
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