🚀 go-pugleaf

In article 338@jyu.fi, sakkinen@jyu.fi (Markku Sakkinen) writes:
>A good point, showing that my suggestion of constant vs. variable
>is worse that value vs. variable as far as regards aliassing problems.
>Thus, there would indeed be some merit in having three parameter modes
>(as somebody suggested in this thread):
>value, constant, and variable

This is exactly what Modula-3 has: VAL, READONLY and VAR.

In article 18702@informatik.uni-ulm.de, borchert@titania.mathematik.uni-ulm.de (Andreas Borchert) writes:

>Readonly parameters don't protect from surprises. E.g. if we have
>the above mentioned
>
>   PROCEDURE p(a: Type; READONLY b: Type);
>   BEGIN
>      a[1] := ....;
>      (* has b[1] been changed now? *)
>   END;
>
>where p is called as p(x, x). And all other cases are still not solved
>where formerly VAR-parameters have been threatened by other procedures
>in a non-obvious way.

What is surprising about this? The semantics of the example is obvious
(at least in Modula-3). b[1] is not changed, of course. a is passed by
value, so the global x (and therefore b) is not changed by the assignment.

Peter
---
Peter Klein                        E-Mail: pk@rwthi3.informatik.rwth-aachen.de
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