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

[Bert Freudenberg]

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 -