Re: Replacing GOTO. [was Re: Conceptual problem solved by GOTO]
Peter Robinson wrote:
>
> Shawn Pringle wrote:
> >
> > Fernando Bauer wrote:
> > >
> > > Suppose we have GOTO and this kind of code (which I think is common):
> > > }}}
<eucode>
> > > procedure proc1()
> > > if cond1 then
> > > .. some code here ..
> > > if cond2 then
> > > .. some code here ..
> > > if cond3 then
> > > .. some code here ..
> > > goto OUT
> > > end if
> > > .. some code here ..
> > > end if
> > > .. some code here ..
> > > end if
> > > OUT:
> > > .. some code here ..
> > > end procedure
> > > </eucode>
{{{
> > >
> > > In Eu 3.1, a possible implementation would be:
> > > }}}
<eucode>
> > > procedure proc1()
> > > for i=1 to 1 do
> > > if cond1 then
> > > .. some code here ..
> > > if cond2 then
> > > .. some code here ..
> > > if cond3 then
> > > .. some code here ..
> > > exit
> > > end if
> > > .. some code here ..
> > > end if
> > > .. some code here ..
> > > end if
> > > end for
> > > .. some code here ..
> > > end procedure
> > > </eucode>
{{{
> > >
> > > Now, without GOTO, we are forced to (mis)use a loop construct (for-loop,
> > > while-loop)
> > > where there is NO loop.
> > > Conceptually, IMHO this makes no sense.
> > > To avoid the conceptual problem we could use flags, but IMHO this is even
> > > worst.
> > >
> > > Regards,
> > > Fernando
> >
> > All non-goto code has been about making code flow as through a tree. Up a
> > trunk,
> > choosing major branches, to smaller branches (routines) and going into twigs
> >
> > (loops, blocks) and then returning through the points until finally getting
> > to
> > a leaf and exiting. This loop idiom to replace goto is something I have
> > forgotten but I did this once. What you see as an argument that goto can
> > make
> >
> > clearer code I see as an argument that there are times we want to implement
> > this kind of control flow but we lack something in the language so we either
> >
> > implement with goto or with a loop.
> >
> > You have avoided spegetti code but that does look like misuse. Goto is also
> > useful for porting from other languages, and there are times
> > you wish to jump to another neighboring twig but do not want to put the
> > overhead
> > of a function call to do it. So, we need goto. Yet, the reason why we have
> > loops and multi-line if/blocks is because it usually easier if we divide the
> > kinds
> > of jumps we are doing into kinds. Is it a loop? Is it an if block? Is
> > it a loop with an integer counter (for)?
> >
> > I feel there is a "missing" kind of structure for putting things in
> > non-looping
> > block and being able to jump out of it as though it were a loop with exit.
> > The
> > one iteration loop works but it isn't a loop semantically speaking.
> >
> > The keywords while, for, and multi-line if blocks exist because people
> > decided
> > that they would replace some usages with goto with control structures that
> > could
> >
> > be easier to read. These are all replacements of usages of goto. The
> > control
> >
> > structure for replaces a usage of goto or of while more directly. We all
> > agree
> >
> > these were good changes even if goto still has a place. I think Fernando
> > has
> > hit
> > on a usage of for which could further be put into a different kind of
> > control
> >
> > structure.
> >
> > So, I want you to think of a new keyword that the developers
> > might decide to put into 5.0 but we mustn't keep changing the target of 4.0
> >
> > or nobody will ever be able to finish it
> > (we will see Starcraft II in stores first, the cows will come home first).
> >
> >
> > Shawn Pringle
>
> I've been thinking along the same lines, except that I don' believe that all
> the patterns can be defined with the precision needed to fix on one, or even
> two, constructs. One of the problems is that the same concept might appear in
> many different forms. You might test the "condition" of sequence data with an
> 'if' block, or loop through its elements, or use a while 1 construct, or just
> a series of top-level code. The most you can say about the pattern is that:-
>
> - A group of constructs execute some testing of data
> - The data either fails early, in which case you want to fall through, but
> maybe
> not right to the end - maybe to an error point,
> - or it triggers a retry, and you want to go back, but quite likely not to
> pass
> through all the same hoops again.
>
> It struck me that part of the lack of structure of the goto comes from the
> single
> label approach, rather than defining a block of code. If you could label a
> block
> of code, it would have more meaning. Just say you chose the keyword 'during'
> with a label. Code like Fernando's would look more structured:-
>
> procedure proc1()
> -- preliminary code
> during data_filtering
> if cond1 then
> -- code here
> if cond2 then
> -- code modifying data
> retry data_filtering
> if cond3 then
> exit data_filtering
> end if
> -- code here
> end if
> -- code here
> end if
> end data_filtering
> -- clean_up code
> end procedure
>
> This really does nothing more than labelled jumps, but the labelling of the
> code as a meaningful block makes it more structured.
>
> Of course, someone will say you could just wrap the innards in another
> procedure
> and use return to exit, and something else for retry, but that doesn't detract
> from Fernando's and your point that the language lacks a construct to
> represent
> a common logical pattern.
>
> And like you, I won't be holding my breath for something like this to appear
> in the language, which is why I'm settling for the current proposal.
>
> Cheers
> Peter Robinson
I think goto still has a place, but I would agree to a block construct such as
suggested. I would prefer block/end block to during though I think.
Aside from creating a place to break out from without abusing the intent of
other constructs, it would provide a way to create nested private scopes.
In my opinion all the various types of code blocks should be able to be
targettable break points, not just loops. A block construct would be a universal
solution.
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