Unexpected behavior
Do the following and see what happens:
- Create a subclass of nil called Test.
- Implement doesNotUnderstand: as ^self.
- Evaluate Test new ifTrue: [4] ifFalse: [5].
Tuesday, March 04, 2008
Subscribe to:
Post Comments (Atom)
About Me
Conferences and Meetings
Smalltalk books available
Books in the works
- Fundamentals of Smalltalk Programming Technique vol 2 [unfinished]
- Fundamentals of Smalltalk Programming Technique vol 3 [unfinished]
- Apuntes Selectos de Matematica [unfinished]
- Stress Resistant Software [unfinished]
- Proven Software Development Process Practices [unfinished]
- AGC is Garbage Collect [unfinished]
- Design Patterns Revisited [unfinished]
6 comments:
Heh, something funny was bound to happen. Seasoned Smalltalkers know that the system cheats at the level of some special selectors, #ifTrue:ifFalse: is one of those.
If you put the code into a method of Test the behaviour will stay the same, then if you use the inspector to drill down into the methodDictionary of the class you can see the byte code of your method which will explain it all ;-)
Let's not forget that mustBeBoolean is answered by the object via DNU before jump false executes.
Andres.
#mustBeBoolean is only sent by the simulator (e.g. when stepping with the debugger).
In my compiled method I see no send of it, and in my virgin 7.6 image the only sender of #mustBeBoolean is Context>>jump:if:.
(I could post the byte codes here, but I don't want to spoil this exercise for others ;-)
I spoke too soon: I went over the VM sources and just learnt that the JIT generates an isBoolean check for the conditional branch byte codes, so it is natural that #mustBeBoolean is not visible in the byte code.
Technically, I do not feel the VM is cheating... jumpFalse could be implemented (in Smalltalk) as
false == thisContext topOfStack
and in that case, we would still have the same behavior without hitting DNU in Test.
Andres.
Not quite... the DNU is interacting weirdly with the upcalled mustBeBoolean... try this code:
Smalltalk.Core defineClass: #Test
superclass: nil
indexedType: #none
private: false
instanceVariableNames: ' '
classInstanceVariableNames: ''
imports: ''
category: 'Kernel-Classes'.
Core.Test compile: 'doesNotUnderstand: message ^false '.
answerOne := Core.Test new ifTrue: [4 ] ifFalse: [5] .
Core.Test compile: 'mustBeBoolean ^false '.
answerTwo := Core.Test new ifTrue: [4 ] ifFalse: [5] .
answerOne -> answerTwo
Post a Comment