On Fri, 4 Jun 1999, Maneesh Yadav wrote:
> Ok...I promise this my last post that I use to correct the previous one.
No biggie. I'm not too fond of Pine either. ;)
> Jim, as far as I cant tell Prism isn't anytning at all except some
> renaming of terms that are standard in a normal imperative OO language.
Well, if that's what you think, then I've clearly done a poor job of
explaining myself. I think that's because I've tried to give too
much background. Let me try again:
Prism is a specification for a programming environment. It specifies:
1) a meta-metamodel (for uniformly modeling concepts such as programs)
2) a global heap (for containing models)
3) an interpreter (for transforming models)
It's nothing like C++. C++ is a language specification; Prism is
not. There may be some superficial similarities (such as Prism's
meta-metamodel and C++ variables both being used to represent
information), but Prism and C++ have nothing in common.
Prism's goal is to allow the programmer to use multiple languages to model
a program. In my last message, I gave an example of this, but it wasn't a
very good one.
I've thought about it some more, though, and I've come up with an example
that is much different from C++ AND solves a real-world problem: dynamic
web serving.
Dynamic web-page serving is a difficult software engineering problem
because there are two very different groups of people working on a
project: programmers, who create the web-page infrastructure, and web
designers, who create the web-page content.
Each group needs a specialized language to do its task. Programmers need
to use a general-purpose imperative language like Perl. Web designers
generally don't understand programming and need to work with HTML markup.
This leads to a problem, because when you're doing dynamic web serving,
the programs have to deliver the HTML.
Since the programs must deliver HTML, naive programmers hard-code the HTML
into their program in the form of strings. This is a huge problem,
because whenever the layout of the page needs to be changed (something the
marketing department likes to ask for), the programs have to be
re-written. And the web designers don't know how to program. So they
layout their pages with HTML, and then the programmers get pulled in to
transcribe the HTML into the program.
Ideally, the team would use two languages: a page markup language for the
web designers, and a general-purpose language for the programmers. Sound
familiar? It should, because that's the kind of task Prism is designed
for.
Here's an example. Two, actually. The first example is written in
Java, and shows how a general-purpose language would be used. The second
example is fictional and shows how some web-optomized Prism languages
might be used to do the same thing.
--- Java example ---
public class ClockPage extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws IOException, ServletException {
resp.setContentType("text/html;charset=ISO-8859-4");
PrintWriter out = resp.getWriter();
out.println("<HTML>");
out.println("<HEAD><TITLE>Time</TITLE></HEAD>");
out.println("<BODY>");
String name = req.getRemoteHost();
if ((name == null) || (name.equals("")) {
name = req.getRemoteAddr();
}
out.println("<P>Hello! You're reading this from " + name + "!";
Calender today = new Calender();
today.setTime(new Date());
int monthInt = today.get(MONTH);
String month = {"January", "February", (etc)}[monthInt];
int dayInt = today.get(DAY_OF_WEEK);
String day = {"Monday", "Teusday", (etc)}[dayInt];
int date = today.get(DAY_OF_WEEK_IN_MONTH);
out.println("<P>Today is " + day + ", " + month + " " + date + ".");
out.println("</BODY>");
out.println("</HTML>");
}
--- Prism example ---
** use Prism/TemplatedHtml **
Time
Hello! You're reading this from ${name}!
Today is ${today.day}, ${today.month} ${today.date}.
** use Prism/WebJava **
public name {
if ((name == null) || (name.equals("")) {
name = req.getRemoteAddr();
}
class date {
public String month;
public String day;
public int date;
init {
Calender today = new Calender();
int monthInt = today.get(MONTH);
month = {"January", "February", (etc)}[monthInt];
int dayInt = today.get(DAY_OF_WEEK);
day = {"Monday", "Teusday", (etc)}[dayInt];
date = today.get(DAY_OF_WEEK_IN_MONTH);
}
}
--- End of examples ---
Even for this simple problem, I think the advantage of using languages
designed for the problem is obvious. Web serving isn't the only situation
where this occurs, either... any complex application is going to be
composed of multiple parts, each dealing with a separate problem. Here's
some of the different things you'll find in applications today:
graphical user interfaces
distributed processing
database access
business models
3D graphics
... and much more.
All of these things can be done with a general-purpose language and
libraries, but I contend that they can be done easier and quicker (and in
some cases, by non-programmers) if specialized, interoperable languages
are used.
That's what Prism is for.
Jim
Prism is at http://www.teleport.com/~sphere