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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Please Help.
Date: 1997/09/13
Message-ID: <dewar.874165401@merv>#1/1
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References: <01bcbcde$f8a425c0$ca70fe8c@default> <dewar.873989183@merv>
 <5vaude$q20$1@goanna.cs.rmit.edu.au> <5vbiid$luu$1@goanna.cs.rmit.edu.au>
 <34197042.7CFC@gsfc.nasa.gov>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-09-13T00:00:00+00:00
List-Id: <comp.lang.ada>


Stephen says

<<Unbounded strings are very nice for beginners. However, they do require
some form of garbage collection to be used correctly. I hope that later
in the course, this will be pointed out.

Which leads me to a question about the GNAT implementation of Unbounded
Strings; do they have their own storage pool, and does Free compact the
pool? One of these days I'll try to read the source.>>


When we say that something is "nice for beginners", we are presumably saying
that students who want to write programs to do X will find it convenient to
use feature Y. But always remember that when we teach computer science,
writing programs as excercises is not about writing programs to do X, it is
about learning the fundamental concepts, and techniques. The programs we write
in class are a *means* to an end, not an end in themselves. This is a very
improtant difference. In the world of real life application programming, the
point is to create a working program that meets the specifications. When we
are teaching the point is to LEARN HOW to create!

Things that make it easier to create a program may or may not facilitate
learning how to create a program. 

Note that this distinction is closely related to the issue of how best
to help students who ask questions. Very often a student writes a note
to CLA saying "I am having trouble doing X", and helpful people assume
that since the student is trying to do X, they should help the student
do X. But that's wrong, the student is trying to *learn* the skills so
that they can do X on their own. Showing them directly how to do X can
often be counter-productive. Of course students don't clearly perceive
this important distinction, because the achievment of X is how their
progress in learning *how* to do X is judged.

As to your technical question (gosh, a technical question on CLA, I had
forgotten that this could happen :-) GNAT uses controlled types to 
implement unbounded strings. Using a special garbage collected pool is
an interesting idea which we have thought about (and which has been discussed
at length in previous CLA threads), but we have not yet followed up this
idea.