How does whileTrue: works in Smalltalk?

Question

I want to know how whileTrue: works. I searched the implementation that is in BlockClosure:

whileTrue: aBlock 
 ^ [self value] whileTrue: [aBlock value]

And another implementation with no parameter:

whileTrue
 ^ [self value] whileTrue: []

But I don't know how that works, that's a recursive method, but this led me to ask several questions:

1. How does this recursive call terminate?
2. If [self value] returns a Boolean object, why is whileTrue: not implemented in the Boolean type?
3. Why is there another implementation named whileTrue that just do not receive any block and evaluates self?

Answer 1

whileTrue: is inlined by the compiler, that's why you don't see the actual implementation. Here's the comment out of Pharo:

"Ordinarily compiled in-line, and therefore not overridable. This is in case the message is sent to other than a literal block. Evaluate the argument, aBlock, as long as the value of the receiver is true."

If you look at the byte code of #whileTrue: you'll see that the compiler simply uses a jump to make the loop:

17 <70> self
18 <C9> send: value
19 <9C> jumpFalse: 25
20 <10> pushTemp: 0
21 <C9> send: value
22 <87> pop
23 <A3 F8> jumpTo: 17 <---- here's the jump
25 <7B> return: nil

#whileTrue is also inlined directly (slightly optimized byte code). The inlining is also the reason why there's no implementation of this on Boolean (apart from the fact that there's no point in reevaluating a boolean...).

#whileTrue is there so that you can easily create endless loops the only terminate e.g. when the process terminates, a semaphore is signaled, an exception occurs etc.

• Lucas Gabriel Sánchez: How can you see the bytecode?
• Max Leske: Which dialect / IDE are you using? In Pharo / Squeak there's a button next to the text area that you can press.
• Lucas Gabriel Sánchez: I'm using Squeak. I will search that button. Thanks.
• Max Leske: In Squeak the button is the right most one above the text area, labelled "source". Click on it and then select "byteCodes".

Answer 2

My Smalltalk is a bit rusty, and I don't have Smalltalk installed at the moment, so there might be some syntax errors in here, but you get the idea:

whileTrue: aBlock
 ↑ self value ifTrue: [aBlock value. self whileTrue: aBlock]

Here's a version in Ruby that I actually tested, of which the above (I hope) is a translation:

class Proc
 def vhile(block)
 self.().iff(thn: -> {block.(); vhile(block)})
 end
end

You need the following monkeypatches to get the Smalltalk-style conditionals to work:

class TrueClass
 def iff(thn:, els: ->{})
 thn.()
 end
end
class FalseClass
 def iff(thn:, els: ->{})
 els.()
 end
end