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From: bobdi@omepd (Bob Dietrich)
Newsgroups: comp.lang.pascal
Subject: Re: The gaping hole left by var parameters ...
Summary: Ain't nuthin new
Message-ID: <3499@omepd>
Date: 26 May 88 18:00:52 GMT
References: <5879@uwmcsd1.UUCP>
Reply-To: bobdi@omepd.UUCP (Bob Dietrich)
Organization: Intel Corp., Hillsboro, Oregon
Lines: 70

In article <5879@uwmcsd1.UUCP> markh@csd4.milw.wisc.edu (Mark William Hopkins) writes:
>What's wrong with var parameters?
>Try these out to see how they compile and run:
>Enjoy ...

I see you've rediscovered aliasing and functions having side-effects, but
you're certainly not pointing out anything new. Languages that have a
variable parameter mechanism and scoping have always had this problem, and
it isn't likely to go away. Likewise functions with side-effects. Both
mechanisms have their drawbacks, but are generally too useful to plug all the
holes (the old "double-edged sword" argument). Attempts are made from time to
time, but they usually just force the programmer into an alternate and
perhaps less natural construct (like returning a result in a parameter
instead of as a function result).

Reminds me of a Pascal compiler brought out a few years ago by a
semiconductor company (not my current employer). The implementors had
read that functions with side-effects are evil, and so their functions
disallowed side-effects (Hmmmm. Did they disallow all file operations? I
can't remember). Anyway, it turns out they also distended procedures so that
they could return values just like functions. So if you wanted a function with
side-effects, you had to call it a procedure returning a value. Someone wanted
good marketing copy, but also saw a need to be filled.

See the Pascal Validation Suite for many dandy examples of aliasing and
side-effects.

> [...]
>--------------------------------------------------------------------------------
>      function ForLoop(var I, Bound : integer) : boolean;
>      begin
>	 ForLoop := (I < Bound);
>	 I := I + 1
>      end;
>
>      ...
>
>    begin
>       I := 0; while ForLoop(I, 3) do A[I] := 2*I;
>       I := 0; while ForLoop(I, 3) do write(A[I]:1, ' ');
>       writeln
>    end.
>
>Why even use a for loop with this dandy available?

I sure hope this fragment doesn't make it through any purported Pascal
processor you have! For the invocations of ForLoop to work, the second
parameter should be a value parameter, and the declaration as follows:

      function ForLoop(var I: integer; Bound : integer) : boolean;

As far as the question goes, when I actually DO use a for-statement, I want
to use it for other than counting integers upwards. I'm just as likely to
use a for-statement with an enumeration, subrange, or the char type. My
programs have few if any variables declared as integer; instead, I use
subranges. I don't know how the variable A is declared in your fragment, but
it looks like its index type begins at one; therefore, any variable used to
index A should probably have a lower bound of one as well. Since Pascal uses
name type compatibility, your function would have few if any uses in any of
my programs.

You might also note that a function like this will get errors if the counter
variable is at the last value of its type (e.g., "ForLoop(I, maxint)").

				Bob Dietrich
				Intel Corporation, Hillsboro, Oregon
				(503) 696-4400 or 2092(messages x4188,2111)
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