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Date:	Tue, 5 Aug 2003 01:48:33 PDT7
From:	"Gregory W. Smith" <gsmith@well.com >
Reply-To:	scoug-programming@scoug.com
To:	scoug-programming@scoug.com
Subject:	SCOUG-Programming: Re: Warpstock 2003 Presentation

Content Type: text/plain

> Lynn H. Maxson wrote:
> > Suppose that you had a variable-length, character variable
> > which not only had an upper limit but whose use dictates a
> > lower one as well. You currently have no means in an HLL
> > to express this rule. Thus you, the programmer, has to
> > enforce the rule (assuming, one, that you know it, and,
> > two, that you remember it wherever) in every instance of
> > its use.
> >
> > You cannot, for example, declare it as part of the data
> > definition, which allows you only to specify an upper
> > limit:
> > dcl jimmy char (17) varying;
> > Now how much of a jump to rocket science is it to be able
> > to change this to
> > dcl jimmy char (5:17) varying;
> > Suppose if it is text, that it contains caps only:
> > dcl jimmy char (5:17) varying range ((A...Z)...);
>
> This is a much better example. I believe that Pascal has the capability
> of defining a variable as a member of a set (for example the only legal
> values are the characters "A, "X" and "Z" or the values (not strings!)
> Dog and Cat) but I've never used that capability of the language so
> can't comment on its robustness.
>
> The ability to define a minimum length is enticing. In PL/I (at least
> in the now-antiquated PL/I Level F compiler) every variable had an
> associated control block which contained the declaration for the
> variable (including the address of the actual data). When you called a
> subroutine the address of the control block rather than the address of
> the data was passed to the subroutine; the subroutine in turn could
> determine what form the data existed in and could act accordingly. You
> could, for example and if the language were extensible, add a BCD value
> to a text string containing Roman numerals. This was typically
> accomplished with a single subroutine containing multiple entry points,
> one entry point for each possible combination of parameter declarations
> plus a general-purpose entry point that would convert the parameters to
> a common declaration.
>
> So it's been done. In assembler. :)))

I will lead off by saying: I don't grok object-oriented programming.
BUT... Isn't the assembler example that Peter pulled out of the guts
of the PL/I compiler really just an example of object-oriented programming?
One that is implemented by the compiler designer behind the scenes?
An "Object" accessed through the control block in assembly?

In spite of Lynn's praises of PL/I, we are now presented with a short
comming of the language. We can only specify an upper limit on the
length of a character variable:
dcl jimmy char (17) varying;
If we want a lower limit as well, THEN JUST CHANGE THE LANGUAGE:
dcl jimmy char (5:17) varying;

I thought the whole point of object-oriented programming was to allow
the programmer to define a class, lets call it CHAR, that would have
the properties of minimum AND maximum length character variables.

But, I am still trying to absorb the Object Oriented aspects of
Python. And it is still a big undigested lump slowly moving down
the snake....

Anyway, take this for what it is worth from someone who does NOT
yet grok this whole Object-Oriented approach.
--
Gregory W. Smith (WD9GAY) gsmith@well.com

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