[Newbies] Re: [] whileTrue: [] implementation

[karl ramberg]

On Wed, Oct 5, 2011 at 4:05 PM, Bert Freudenberg <bert at freudenbergs.de> wrote:
> On 05.10.2011, at 00:24, nicolas cellier wrote:
>
>> Bert Freudenberg <bert <at> freudenbergs.de> writes:
>> On 04.10.2011, at 19:16, Erlis Vidal wrote:
>>>
>>>> Thanks Bert!
>>>>
>>>> The byte code has the answer, I thought that the byte code will map the
>> source code but I see that's not the
>>> case, probably to ensure some optimizations.. or to do that "jump" I don't
>> know how to do that in smalltalk
>>>
>>> For ifTrue: it is purely an optimization, yes. It would work fine by actually
>> sending that message. It would
>>> just be slower.
>>>
>>> In the case of whileTrue:, however, it's not just an optimization. The only
>> way to do loops otherwise would
>>> be by recursion. But the Squeak VM does not do tail-call elimination, so it
>> would run out of space pretty
>>> soon. Generating the loop as a byte code back jump is essential for the system
>> to work.
>>>
>>> - Bert -
>>>
>>
>>
>> Oh no, we could as well reduce stack depth to (numberOfIteration log: 2) with
>> silly recursive code like this:
>>
>>
>> Block>>tryTwiceThen: aBlock
>>       self value ifFalse: [^aBlock value].
>>       self value ifFalse: [^aBlock value].
>>       ^
>>       [self value ifFalse: [^aBlock value].
>>       self value ifFalse: [^aBlock value].
>>       true]
>>               tryTwiceThen: [^aBlock value]
>>
>> Block>>log2WhileTrue
>>    ^self tryTwiceThen: [^nil]
>>
>> | i maxDepth sender |
>> i := maxDepth := 0.
>> [i := i+1.
>>       depth := 0.
>>       sender := thisContext.
>>       [(sender := sender sender) isNil] whileFalse: [depth := depth+1].
>>       maxDepth := maxDepth max: depth.
>> i<10000] log2WhileTrue.
>> ^maxDepth
>
> Yes, but log(n) is still not good enough for loops that run forever. Like the Morphic main loop. Or the idle process.
>
> I don't think it's possible to emulate whileTrue without reaching for the meta hammer. Interesting discussion though. Maybe a bit over the head of the average newbie ;)
>
> - Bert -

But this kind of thread is also one of the reasons I like the Squeak
community. Newbies get right to the core with their questions and
there is little  hierarchy.

Karl