Re: [Help-smalltalk] ContextPart>>printOn: prints incorrect line

[Ladislav Marek]

Thanks. Nice explanation.

On Fri, Jul 15, 2011 at 09:52, Paolo Bonzini <address@hidden> wrote:
> On 07/14/2011 08:25 PM, Ladislav Marek wrote:
>> I have observe strange behavior of MethodContext>>printOn: and
>> ContextPart>>currentLineInFile
>>
>> Eval [
>>       thisContext print.
>>       1.
>> ]
>>
>> This code outputs: UndefinedObject>>executeStatements (test.st:3),
>> line is incorrect, I think it should be 2.
>>
>> Eval [
>>       thisContext currentLineInFile printNl.
>>       1.
>> ]
>>
>> This code outputs: 2, as expected. I look at the
>> MethodContext>>printOn: method and there is
>> ContextPart>>currentLineInFile called, so why it outputs different
>> line number?
>
> It's an off-by-one error.  This is the compiled bytecode for your first
> example:
>
>    0:  source line 2
>        push Global Variable[0] = ContextPart
>    2:  send special message #thisContext
>    4:  send selector 1, 0 args = #printNl
>    6:  source line 2
>        pop stack top
>    8:  push 1
>        return stack top
>
> The instruction pointer when sending #printNl is already 6.  The call to
> #currentLineInFile prints erroneously takes the "source line" bytecode at
> address 6 into account, and prints 2+2-1 = 3.
>
> This patch fixes it:
>
> diff --git a/kernel/ContextPart.st b/kernel/ContextPart.st
> index bfcf8d5..dc11dd3 100644
> --- a/kernel/ContextPart.st
> +++ b/kernel/ContextPart.st
> @@ -125,7 +125,7 @@ methods that can be used in inspection or debugging.'>
>         thus making the implementation faster."
>
>        <category: 'debugging'>
> -       ^self method sourceCodeMap at: self ip + 1 ifAbsent: [1]
> +       ^self method sourceCodeMap at: self ip - 1 ifAbsent: [1]
>     ]
>
>     debugger [
>
> Thanks for reporting it!
>
> Paolo
>