Re: VOTE - thanks James.
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(a)sbcglobal.net>