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HELP POP-11                                      Robert Duncan, May 1995

An introduction to the Poplog Pop-11 subsystem.

         CONTENTS - (Use <ENTER> g to access required sections)

  1   The Pop-11 Language

  2   Running Pop-11

  3   Writing a Program

  4   Initialisation and Libraries

  5   Command Line Arguments

  6   Additional Information

1  The Pop-11 Language

Pop-11 is the core language of the Poplog system. Most of Poplog itself
is written in Pop-11, including all the subsystem compilers, the editor
and the X interface. For this reason alone, it is worthwhile for all
Poplog users -- whatever their primary language -- to have some
familiarity with Pop-11, since there are inevitably some features of
Poplog which can most easily (or perhaps only) be accessed through
Pop-11 by means of mixed-language calls.

Pop-11 has evolved from a long line of Pop languages, developed
primarily for applications in Artificial Intelligence. Unsurprisingly,
then, it shares many features with modern Lisp languages, features which
emphasise the importance of symbolic programming, data abstraction,
convenience of use and language extensibility. But, as with Lisp, the
usefulness of Pop-11 extends far beyond the traditional AI domain: those
same features which make the language suitable for tackling the complex
problems posed by AI also make it a powerful tool for solving hard
problems in all kinds of application areas. The use of Pop-11 within
Poplog serves as an immediate example of its general applicability.

Characteristic features common to the Pop-11/Lisp style include:

  # built-in support for a varied range of datatypes. Types built in to
    Poplog Pop-11 include: strings, words (symbols), booleans, a rich
    set of numeric types, aggregates (lists, vectors and arrays),
    generic mappings (properties) and various system types including I/O
    devices and ``lightweight'' processes

  # the ability to add new, user-defined types with the same status as
    those built-in

  # first-class procedures with a choice of lexical or dynamic scoping
    of variables

  # a dynamic (run-time) type system

  # automatic storage management (garbage collection)

  # an interactive programming environment

  # an extensive collection of library procedures

  # extensible syntax

But the Pop tradition has always rejected Lisp-style syntax in favour of
a more conventional, structured style. By way of example, here is the
definition of a Pop-11 procedure which computes the value of a
polynomial of arbitrary degree, given a list of coefficients and a value

    define polynomial(x, coefficients) -> y;
        lvars c, y = 0;
        for c in coefficients do
            y * x + c -> y;

    ;;; print the value of 11x^3 + 4x^2 - 5 for x = 7
    polynomial(7, [11 4 0 -5]) =>

Most newcomers to the language find this core syntax straightforward
both to read and write and can move on quickly to developing significant
programs, concentrating on semantic issues without being distracted by
syntactic details. Experienced programmers can build upon the core
syntax, using its extensibility to define new forms tailored to their
own problem domains. This extensibility also allows Pop-11 to develop
incrementally over time as experimental features become accepted and
integrated into the mainstream language.

A second aspect of Pop-11 which distinguishes it from Lisp is its
explicitly stack-based evaluation semantics. This makes Pop-11
programming a distinctly novel experience for many users, and can
provide some elegant programming solutions. Here, for example, is a
simple procedure which selects interesting items from a list:

    define select(list, interesting) -> selected;
        lvars selected = [%
            lvars item;
            for item in list do
                if interesting(item) then item endif;

If we define as ``interesting'' numbers which are integral, then the

    select([6 13 5 27.42 31 9.45 31.24], isintegral) =>

will display the list

    [6 13 5 31]

It is the implicit use of the stack in select which allows the
evaluating list brackets ([%%]) to collect an arbitrary number of
results. As an aside, this example also demonstrates the regular
treatment of procedure values in Pop-11 -- as in Scheme and other
functional languages -- exemplified here by the binding of the formal
parameter interesting to the built-in procedure isintegral.

Use of the stack is often transparent, and it's possible to write Pop-11
in a very clean, functional style. Some users, however, choose to use
the stack aggressively, adopting a style not unlike that of Forth. This
is merely an example of how the richness of Pop-11's syntax and
semantics can support a variety of programming paradigms which in other
contexts might appear to be in conflict. Its extensibility means it can
be continually adapting to new modes and styles of programming, most
recently -- in line with current trends -- towards an object-oriented

The Pop-11 language is regrettably less well-known than it ought to be.
Anyone wishing to begin experimenting with the language should start by
reading TEACH * PRIMER which gives an overview of many of the language

2  Running Pop-11

From the operating system prompt, the pop11 command will take you into
the interactive top-level; your terminal screen should look something
like the following:

    Sussex Poplog (Version 15.03 Thu Jun 20 16:20:20 BST 1996)
    Copyright (c) 1982-1996 University of Sussex. All rights reserved.


The Setpop message is printed whenever the Pop-11 compiler is started:
here for the first time and subsequently after an error or interrupt.
The colon (:) is the Pop-11 prompt, and indicates that the compiler is
ready for input: you can type any Pop-11 expression or command here,
such as those copied from above. A good starting point is to use the
immediate-mode command:

    : im

which will start an interactive session within an editor window. This
works just like the top-level interaction, but you can use editor
commands to edit and re-enter input and format output for easier
reading, or to save a record of the interaction to a file.

Other useful commands to try are:

    : help topic
        Displays documentation on the given topic

    : load file
        Compiles a file

    : ved file
        Edits a file

    : bye
        Terminates the interaction: at top-level, this will exit from
        the pop11 command

You can use all these commands from inside the editor too, by using the
<ENTER> key to take you to the editor command line.

X users should start pop11 with the %x command-line argument: in a
standard system this will pop up a ``control panel'' window from which
you can invoke common Poplog functions. Browse through the Help menu to
find information about using the control panel and the editor or choose
the Interaction option from the File menu to start an interactive

3  Writing a Program

Use the ved command to open a program file or choose Open from the
control panel File menu. By convention, Pop-11 program files have an
extension of '.p'. If you conform to this, Poplog will recognise your
program files and assign to them the correct compiler. A program can
contain a mixture of definitions and execute-level evaluations: you
could copy exactly the program fragments shown above, for example.

You can compile a program from within the editor by using the

    <ENTER> l1

command, where l1 means ``load one file''. This compiles directly from
the current editor buffer, so you don't need to save the file first. Any
output or errors will be sent to an editor output window.

To compile a named file, use the load command, either at the interactive
prompt or from the editor command-line. Alternatively, use the Compile
option from the control panel File menu.

The Pop-11 compiler is incremental. This means that regardless of which
files you compile or how you compile them, all the definitions they
contain -- unless you have explicitly chosen to restrict their scope --
are added to a global environment shared by all other program files and
interactive windows. So you can break a program across multiple files as
convenient and test out individual procedures interactively. Modifying
and recompiling a file updates the environment to reflect any new or
changed definitions, allowing you to switch quickly between writing and
testing code, and to build up a complete program in incremental steps.
As your files get larger, you can use editor commands to compile just
part of a file, omitting those sections that aren't yet completed or
which have been compiled once and not since modified: TEACH * LMR and
TEACH * LCP tell you how to do this. This leads to a highly productive
method of working.

4  Initialisation and Libraries

When starting in interactive mode, pop11 will compile an initialisation
file called "init.p" if it exists in your poplib directory. By default,
this directory is assumed to be your home (login) directory. You can
choose a different location by setting poplib as an environment variable
(logical name) before starting pop11; this is most conveniently done in
your ".login", ".profile" or "" file.

The "init.p" file is a standard Pop-11 program file, so can contain any
code you like for customising your working environment. Clearly, the
more you put in here the longer it takes to compile, increasing the time
it takes to get to the interactive prompt. As you develop a collection
of frequently-used procedures to which you want ready access, rather
than define them all in your "init.p" file, you should consider making
them into libraries which you can load explicitly as needed.

This is easy to do. First, create a library directory. The pathname


will do as an example; VMS users might prefer to write this as


but Poplog will convert automatically from the Unix form to maximise

Define your procedures in one or more '.p' files in this directory, e.g.


and add the line

    '~/pop11/lib/' :: popuseslist -> popuseslist;

into your "init.p" file. This adds the library directory to the library
search path; to load the useful procedures, you need only type

    lib useful;

More sophisticated effects are possible with the uses syntax and
autoloadable identifiers: see REF * LIBRARY for details.

The %noinit command-line option prevents compilation of the "init.p"
file. This is useful for quick startup, or when your init file contains
some kind of error.

5  Command Line Arguments

To summarise the command-line syntax discussed in this file: the general
form of the pop11 command is

    pop11 [%args] [vedcommand] [file]

All arguments are optional. The %args control system startup behaviour:

    %x ( X Toolkit options )
        Initiates a connection with the X server by calling *sysxsetup.
        You can supply standard X Toolkit options by placing them in
        parentheses after the %x.

        Suppresses compilation of the "init.p" file and any init files
        used by Ved or other subsystems.

A vedcommand argument causes Poplog to go straight into the editor, and
can be any of the following:


Used in conjunction with a vedcommand argument, a trailing file name is
given as argument to that command. Otherwise, if there is no Ved
command, the file is compiled as a Pop-11 program file; in this case,
your "init.p" file is not compiled, regardless of whether %noinit was

Instead of a file, you can give an argument


in which case the expression is evaluated by the Pop-11 compiler.


    pop11 %x im
        Starts up Pop-11 with an X connection, displaying the user-
        interface control panel and an immediate-mode XVed window

    pop11 ref library
        Starts up Pop-11 with a Ved window displaying REF * LIBRARY

    pop11 ":popversion=>"
        Prints the Pop-11 version string

Full details of the command line are given in REF * SYSTEM.

6  Additional Information

On the Pop-11 language:

        An introduction to Pop-11 with many examples

        Itemisation and syntax rules

        Built-in types

        Equality and pattern matching

        Object-oriented extensions

        Input and output

        Advanced features

On Poplog in general:

        Startup and exit, saved images, storage management, etc.

        Subsystems and libraries

        Variables controlling system behaviour

        Other literature relevant to the Poplog languages

--- C.all/help/pop11
--- Copyright University of Sussex 1996. All rights reserved.