OpenEuphoria

Re: EOF and GOTO and 0th array element

• Posted by Juergen Luethje <jluethje at gmx.de> Aug 23, 2002
• 488 views

Hello Derek,

Derek <ddparnell at bigpond.com> wrote:

<snip>
> I will argue against the "exit n" idea for a number of reasons.
>
> The first reason is that over time, the value of 'n' might change as
> modifications change the nesting level of the 'exit' statement. This means
> that one would have to examine, and possibly update, all the 'exit n'
> statements whenever you add or remove a loop block. Instead of 'exit n', I
> would suggest that 'exit name' is a safer and less maintenance intensive
> alternative. The reason being that the name (which can only be attached to
> the start of a loop block), does not change when one adds and deletes loop
> blocks. Thus any reference to the name does not have to be examined. Another
> reason I prefer this is that a descriptive name can aid the reader as to
> what the code is trying to do, whereas a number doesn't do this.
>
> So in summary, I propose that the syntax for 'while' and 'for' loop blocks
> be changed to ...

>   [<NAME>:]while <BOOLEANEXPR> do <STATEMENTS> end while
>   [<NAME>:]for <FORCONTROL> do <STATEMENTS> end for

> and the <STATEMENTS> may include one or more ...
>    exit [<NAME>]
> where if no name is specified, the loop block that the exit is in is
> terminated. If a name is specified, the named loop block is terminated. If
> there is no loop block thus named, an error is generated.

Now, having read your explanations, and having thought about the problem
more in depth, I'm convinced that your proposal for enhancement of the
exit statement is really better than "exit n".

> I refer to this idea as a controlled GOTO. Firstly, it must be mentioned
> that the existing 'exit' statement is also a controlled GOTO. It passes
> control to the end of a loop block. It is controlled in the sense that the
> target of the exit is not any arbitary line of code. It is ALWAYS the first
> line after the loop block. My proposal is no different in concept. Control
> is always passed to the first line of code after the named loop block.

Well, in this sense, even an "if/end if" statement could be called kind
of "goto". That's not what I meant. My previous remarks about GOTO were
only meant to refer to the explicit GOTO statement, that is provided by
BASIC and other languages, and that obviously some people would like to
see in Euphoria.

<snipped in agreement>

>> <quote>
>> Many authors have suggested language features [...]
>> which are expressed in terms of *exit*, *jump-out*,
>> *break*, or *leave* statements. Kosaraju [57] has
>> proved that such statements are sufficient to
>> express all programs without *go to*'s and without
>> any extra computation, but only if an exit from
>> arbitrarily many levels of control is permitted.
>> <unquote>
>> [Knuth, Donald E.: Structured Programming with go to Statements.
>>  Computing Surveys, Vol. 6, No. 4, December 1974, pp. 261-301]

> Hey, if the Master says so, its okay by me too! :) Notice though that the
> idea of "arbitrarily many levels of control" is not necessarily expressed in
> terms of Number of Levels.

I wrote this quote as an argument against the GOTO statement (in the
sense I explained above), not as an argument against your "enhanced exit"
proposal.

> ----------------
> cheers,
> Derek Parnell

Thank you for your comprehensive reply.

Regards,
   Juergen