Re: "Subtract C, add Ada"

[mbk@inls1.ucsd.edu (Matt Kennel)]

Jay Martin (jmartin@oahu.cs.ucla.edu) wrote:
: Heh, (C Idiom == Idiotm)

: >Writing ``for (p = head; p; p = p->next) { /* process list */ }''

: Ptr := Head;
: while (Ptr /= null) loop
:   ...
:   Ptr := Ptr.Next;
: end loop;

: (My Ada is rusty, I only program in C++ now)

		"Think Globally, Act Locally."	

All these silly questions come down to the fact that control
structures are hardwired into the language, but not easily
reprogrammable.

Real iterations in real problems come in many forms.  You need a
user-progammable construction that goes straight to the problem.
C++ types 'iterators' are a good first step, but I don't feel
super comfortable with them.  What I think is even better is
Sather's "iters".  Instead of arguing over silly little details
of various Pascal or C control structures or whatever their trivial
variations, we should desire something much better and powerful among
all the inventions that have been occured over the last 20 years.

In particular I know about Sather's iters, which are practical
and convenient.

Example:
--------
list,list2:LIST{BLAH};
loop
  	list_element := list.elt!;  -- iterate through elements of "list".

	process(list_element);

	while!(list_element /= special_gonzo); 
                -- break out if we hit mr gonzo.

	list_element2 := list2.elt!;	
	  -- parallel (not nested) iteration through second list

	process2(list_element,list_element2);
end;


"elt!" is a feature in LIST{BLAH} which returns successive elements of
the list, and breaks out if it hits the end.

This fragment does exactly what it looks like it does.  And the
user of the "list" does not need to know whether it returns elements by
dint of following successive pointers, stepping through arrays, or black magic.

It eliminates stupid "off-by-one" errors and needless special boundary
cases.

cheers
Matt

--
-Matt Kennel  		mbk@inls1.ucsd.edu
-Institute for Nonlinear Science, University of California, San Diego
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