[Oberon] Multiple RETURN in a procedure

[Aubrey McIntosh]

I think that Wirth's example cited by Denisof would be easier to
comprehend if an aux variable were introduced.  I applied this to the min
function being discussed as follows:
  PROCEDURE minTrans (x,y: INTEGER) : INTEGER;
  (*41 bytes*)
    VAR aux: INTEGER;
  BEGIN IF x < y THEN aux := (x) ELSE aux := (y) END;
    RETURN aux
 END minTrans;

More formally, I substituted "aux :=" everyplace "RETURN" existed in the
original snippet, and I placed the "RETURN aux" immediately before the
Function termination.

How difficult would it be to put such a strategy into a OldSource -->
NewSourcee compiler?


On Sun, Oct 23, 2022 at 5:03 PM Chris Burrows <cfbsoftware at gmail.com> wrote:

> On Mon, Oct 24, 2022 at 2:03 AM <thutt at harp-project.com> wrote:
>
>> The original ETHZ compiler didn't check if you included a return
>> statement.  It actually did something that I find to be really nice --
>> something that would be handy for others to adopt:
>>
>>    If you did not have a return statement that was executed in a
>>    procedure returning a value, a trap was raised.
>>
>>
> Better than nothing I suppose, but it is always preferable, if possible,
> for errors to be detected at compile-time rather than runtime.
>
>
>>  I am not a fan of the single return, though it would probably be
>>  beneficial for the GSA IR and code generation.   The return causes an
>>  immediate return from the function, but the assignment to the
>>  procedure name means that the control flow will continue through the
>>  function.  For many non-trivial functions, with a naive compiler,
>>  this will lead to unnecessary code being executed, and perhaps faults
>>  being raised.
>>
>>  Consider:
>>
>>   PROCEDURE ListLength(ptr : List) : INTEGER
>>   VAR r : INTEGER;
>>   BEGIN
>>     IF ptr = NIL THEN
>>        RETURN 0;
>>     END;
>>     r := 0
>>     WHILE ptr # NIL DO
>>        INC(r);
>>        ptr := ptr.next;
>>     END
>>     ListLength := r;
>>     RETURN ListLength;
>>   END ListLength;
>
>
>
> I'm not that familiar with Oberon-07, so maybe there's some nuance I'm
>> missing?
>>
>
> In Oberon-07 this would simply be:
>
>   PROCEDURE ListLength(ptr : List) : INTEGER;
>   VAR r: INTEGER;
>   BEGIN
>     r := 0;
>     WHILE ptr # NIL DO
>       INC(r);
>       ptr := ptr.next;
>     END
>     RETURN r
>   END ListLength;
>
> Regards,
> Chris Burrows
> CFB Software
> https://www.astrobe.com
>
>
> --
> Oberon at lists.inf.ethz.ch mailing list for ETH Oberon and related systems
> https://lists.inf.ethz.ch/mailman/listinfo/oberon
>


-- 
Aubrey McIntosh, PhD
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