Question

Smalltalk has the whileTrue:-Message implemented through recursion (in VisualWorks) or through compiler-inlining (in Squeak/Pharo). Is there a way to define such a method without using one of them? If not, is there a proof for that avaiable somewhere?

Answer 1

whileTrue: & whileFalse: always return nil.
e.g. if there is a normal recursive definition:

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

the ifTrue: will return nil if self value is false and so the value should always be nil. That’s reflected in the compiler’s optimization. The original blue book Smalltalk-80 V2 definition is

whileTrue: aBlock
    "Evaluate the argument, aBlock, as long as the value
    of the receiver is true. Ordinarily compiled in-line.
    But could also be done in Smalltalk as follows"

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

So just change your’s to

BlockContext>>myWhileTrue: aBlock 
    | start |
    start := thisContext pc.
    self value ifFalse: [ ^ nil ].
    aBlock value.
    thisContext pc: start

or??

BlockContext>>myWhileTrue: aBlock 
    | start |
    start := thisContext pc.
    ^self value ifTrue:
        [aBlock value.
         thisContext pc: start]

But alas both of these crash the VM sometime after the second iteration because thisContext pc doesn’t answer the pc on the next iteration, but instead whatever the top of stack is 🙂

However the following does work:

ContextPart methods for controlling
label
    ^{ pc. stackp }

goto: aLabel
    "N.B. we *must* answer label so that the
     top of stack is aLabel as it is when we send label"
    pc := aLabel at: 1.
    self stackp: (aLabel at: 2).
    ^aLabel

BlockContext>>myWhileTrue: aBlock 
    | label |
    label := thisContext label.
    self value ifFalse: [^nil].
    aBlock value.
    thisContext goto: label

BlockClosure>>myWhileTrue: aBlock 
    | label |
    label := thisContext label.
    ^self value ifTrue:
        [aBlock value.
         thisContext goto: label]