VOTE - thanks James.

[Aaron Konstam]

On Mon, 2007-01-01 at 12:26 +0000, James Wilkinson wrote:
> Aaron Konstam wrote (about COMEFROM):
> > This joke is about 30 years old. It has been has been so long since it
> > was first mentioned that it is hard for me to explain clearly why this
> > is indeed not compilable but it is not.
> > I guess if one can't explain something clearly one should shut up. It
> > leads to situations which are logically unresolvable. As someone who has
> > taught compiler construction it should be obvious to me why this is true
> > but my senior moments have caught up with me. If the detailed
> > explanation comes to me I will share it.
> 
> As I understand it (and I'm certainly not a compiler expert):
> 
> Computer languages do have to be well defined. If you have something
> like (pardon the pseudo-BASIC):
> 10 PRINT "Hello."
> 20 COMEFROM 40
> 30 PRINT "How to exercise a CPU in three instructions!"
> 40 PRINT "Normal definitions of COMEFROM say this will never be run..."
> 50 QUIT
> That *is* "compilable" since you can come up with a "valid" binary that
> does what the programmer expects.
> 
> If on the other hand you have
> 10 PRINT "Hello."
> 20 COMEFROM 60
> 30 PRINT "Schrödinger may or may not have been here..."
> 40 COMEFROM 60
> 50 PRINT "but he *was* here!"
> 60 QUIT
> what happens when you get to line 60? Should the program restart from
> line 20 or line 40? With normal, single threaded programs, it's not
> possible to do both, so you can't get a binary that does. Therefore it's
> not compilable (without extra definitions).
> 
> Suggested workarounds include returning to different COMEFROMS at
> random, or starting different threads (or forking new processes), one
> for each COMEFROM.
> 
> James.
Thanks James. I had a response (more complex) worked out but your
example explains the problem quite clearly. I am embarrassed that I drew
a blank in doing this immediately.
-- 
Aaron Konstam <akonstam at sbcglobal.net>