EPL 0.7 Reference Manual

Node:Top, Next:Preface, Up:(dir)

# EPL

This is edition 0.2 of the EPL Reference Manual, corresponding to EPL version 0.7.

EPL (Emacs Perl) is a library of Lisp functions and Perl modules that let you write Emacs extensions in Perl.

- Preface: Why I'm here.
- Introduction: Beginning with an example.
- Features: How Perl looks to Lisp, how Lisp looks to Perl.
- Configuration: Changing the defaults.
- Internals: What happens under the hood.

  Appendices

- Copying Conditions: The GNU Free Documentation License.
- Index: Index to this manual.

  --- The Detailed Node Listing ---

  Introduction to EPL

- Perl Example: An example: Find files containing regex.
- History: EPL uses IPC, not translation or embedding.
- Emacs Lisp: You need the Emacs Lisp Reference Manual.
- Coding Conventions: Why the Perl examples use `&function()`.

  How Language Features Correspond

- Data Conversion: Working with each other's data types.
- Functions: Transfering program control.
- Control Structures: Exceptions; `throw`, `signal`, `die`.
- Input/Output: Perl's primitives are redefined in Emacs.
- Caveats: It's not always a happy marriage.

  Configuration

- Installation: Lisp files, Perl modules, documentation.
- The Easy Way: We like the defaults.
- Starting a Perl Process in Emacs: `make-perl-interpreter`.
- Starting an Emacs Process in Perl: `use Emacs;`.

  Internals

- Debugging Options: Dumping traffic to stderr or a buffer.
- Transport Layer: Media of interprocess communication.
- Message Types: What interpreters can say to each other.
- State Transitions: The message stream grammar.
- Protocol: Current EPL protocol (subject to change).

Node:Preface, Next:Introduction, Previous:Top, Up:Top

# Preface

Like a lot of people, I learned Perl because of the Web. I came to GNU from a Microsoft background early in 1996, when Microsoft had only begun to realize that the Internet was hot. I was deeply infused with Web hype, willing to try something different, and aware that the Web ran on Unix and Perl.

I came to Emacs because of genetic predisposition. A tightly integrated, yet versatile, complex system with endless opportunities for learning and automation helps reduce my cholesterol and, I think, aids respiration and digestion. It had all the good features of Microsoft Office, without all the headaches and nausea.

Well, it had almost all the good features.

Emacs is an oasis of integration in the chaotic and disparate realm of Linux and Unix. (I later learned that the Unix kernel is a thing of some elegance, but at the level of `/etc/resolv.conf`, X11R6, and `~/.fvwmrc`, there was little cohesion.) As I entered this realm, it irked me that I had to learn two scripting languages to do what I wanted, whereas Visual Basic, clunky as it is, does everything on Windows.

Speaking of that other--umm--language, Visual Basic shares an important feature with Perl: It is for ordinary people.

Now, I am very glad that Emacs lets you do just about anything with Elisp (such as write EPL). But my conceptual mapping from Windows onto Unix paired Visual Basic with Perl, not Lisp, as the general workhorse get-it-done tool. It seemed to me not right that the standard, full-featured editor on my new system could not understand the standard, full-featured scripting language. With EPL--Emacs PerL, or Emacs-Perl-Lisp--I hope to rectify the situation.

Node:Introduction, Next:Features, Previous:Preface, Up:Top

# Introduction

EPL is an extension for GNU Emacs and XEmacs that lets you write commands in Perl. It is also a programming interface for Elisp (see Emacs Lisp) programs to use Perl features and an interface for Perl to use Emacs.

This document assumes knowledge of Emacs terms such as "the minibuffer" and "`M-x`" (see Top). It assumes knowledge of Perl references and nested data structures (perlref(1)), and packages and modules (perlmod(1)).

- Perl Example: An example: Find files containing regex.
- History: EPL uses IPC, not translation or embedding.
- Emacs Lisp: You need the Emacs Lisp Reference Manual.
- Coding Conventions: Why the Perl examples use `&function()`.

Node:Perl Example, Next:History, Up:Introduction

## Perl Example

Let's illustrate EPL with an example. Suppose you often want to edit many files in the same directory based on some text that they contain. You want a command that will look for a specified regular expression in each file, and if found, open the file in a buffer. You prefer Perl regular expressions to Emacs ones, and you are not interested in files that Perl considers to be binary.

Here is some Emacs Perl code that will do it:

    use IO::Dir;
    use IO::File;

    sub find_my_files ($$) {
        my ($dirname, $regex) = @_;

        # Convert "~" to home dir.
        $dirname = &expand_file_name ($dirname);

        my $dh = IO::Dir->new ($dirname)
            or die "$dirname: $!";

        my $count = 0;
        while (defined ($filename = $dh->read)) {
            $filename = "$dirname/$filename";

            # Skip directories and binary files.
            next unless -f($filename) && -T($filename);

            my $fh = IO::File->new ($filename);
            if (!$fh) {
                warn ("$filename: $!");
                next;
            }
            while (defined ($_ = <$fh>)) {
                if (/$regex/) {
                    &find_file($filename);
                    $count++;
                    last;
                }
            }
        }
        &message ("Found $regex in $count file(s)");
    }

    defun (\*find_files_containing_regex,
           "Open all files whose contents match a Perl regular expression.",
           interactive("DLook in directory: \nsPerl regex: "),
           \&find_my_files);

You would save this in a file, load it with `M-x perl-load-file` (see Evaluation) or from `~/.emacs`, and use it as `M-x find-files-containing-regex`.

The command prompts for a directory name (with <TAB>-completion and history) and an arbitrary string (regex). It signals an error if it can't read the directory, and otherwise summarizes the results in the minibuffer. You can access its documentation just like that of any Emacs command, using `C-h f find-files-containing-regex`.

Node:History, Next:Emacs Lisp, Previous:Perl Example, Up:Introduction

## History

EPL works by starting a subprocess the first time a Perl interpreter is needed to carry out some function. Emacs and Perl use interprocess communication (IPC) to invoke functions and send each other data until the parent process terminates. Variations on this theme may involve Perl as the parent process or more than one child per parent.

This is not the only possible implementation. In principle, the same functionality could be achieved by translating Perl to Lisp. That has been a goal of the GNU project since 1994, except that RMS wants to replace Elisp with Scheme first. For details, see Lisp History, and http://www.vanderburg.org/Tcl/war/.

Another possibility, which has been realized by a program called Perlmacs, is to embed the Perl interpreter in Emacs and let it operate directly at the C level the way the Elisp interpreter does. This probably results in faster execution, but it ties an executable to specific versions of Perl and Emacs. Perlmacs does not support XEmacs.

Node:Emacs Lisp, Next:Coding Conventions, Previous:History, Up:Introduction

## Emacs Lisp

Whether you write EPL programs or Elisp programs, you are writing Emacs programs. The GNU and XEmacs projects maintain comprehensive and thorough documentation of the functions available to Emacs extensions.

Of course, those documents assume that extensions use Lisp. Still, they are an invaluable resource, and any programmer with a general understanding of functions and variables can make good use of them. This manual makes frequent reference to the GNU Emacs Lisp Reference Manual. If you don't have it, go get it! It should be available wherever you got Emacs or from the Free Software Foundation.

Who knows? You may fall in love with Lisp.

Node:Coding Conventions, Previous:Emacs Lisp, Up:Introduction

## Coding Conventions

Node:Features, Next:Configuration, Previous:Introduction, Up:Top

# Features

This chapter describes EPL's mapping between Perl's language features and Emacs Lisp's. Perl and Lisp are quite different, so some features favored by one language are imperfectly or awkwardly reproduced in the other. EPL follows Perl's philosophy of making simple things easy and hard things possible.

- Data Conversion: Working with each other's data types.
- Functions: Calling each other's functions, `defun`.
- Control Structures: Exceptions; `throw`, `signal`, `die`.
- Input/Output: Perl's primitives are redefined in Emacs.
- Caveats: It's not always a happy marriage.

Node:Data Conversion, Next:Functions, Up:Features

## Data Conversion

EPL provides essentially two mechanisms to transfer data between languages--as function arguments and return values. In both cases, data may be passed either by value or by reference. The default is by value, since references incur overhead (see References Incur Overhead).

In EPL, by value means recursively (or "deeply") copied. That is, containers in the originating language are converted to analogous containers in the destination language, and any contained elements not explicitly marked as pass-by-reference are likewise converted. This allows the recipient to examine a structure using its built-in accessors, but changes will not apply to the original. Also, language peculiarities result in information loss during some conversions, so pass-by-value is not completely reversible (see Conversion Is Imperfect).

This section describes the conversions that apply to each language's scalar and container types. For a complete description of Lisp types, see Lisp Data Types.

- Scalars: Lisp has three scalar types.
- The Null Value: `undef` and `()` are `nil`.
- Symbols and Globrefs: `\*::sym` is `'sym`.
- Lists and Arrayrefs: `["fnord"]` is `'("fnord")`.
- Vectors: `\[42]` is `[42]`.
- Conses: The `Emacs::Lisp::Cons` type.
- Scalar References: The `perl-ref` type.
- Code References: `\&frob` is a lambda expression.
- Hash Tables: Not yet implemented.
- Blessed References: Not yet implemented.
- Emacs Types: Not yet implemented.

Node:Scalars, Next:The Null Value, Up:Data Conversion

### Scalars

Lisp integers, floats, and strings all become Perl scalars. A simple Perl scalar becomes either an integer, a float, or a string.

Unfortunately, it is hard to be certain which type will be chosen. It depends on the value. Perl 5.005 and later can distinguish among the literal constants `1`, `"1"`, and `1.0`, and EPL uses this information. However, not all Perl integers fit into an Emacs integer (which is 28 bits, see Integer Type). Integers that would overflow are upgraded to floats when converted.

You may be able to specify string conversion by wrapping a variable in double quotes (`"$var"`), float by adding zero (`$var + 0`), and integer using (`int($var)`). Perhaps a future version of EPL will provide an explicit means of specification.

Interesting character encodings such as UTF-8 are not currently supported. All strings are considered unibyte. See Text Representations.

Node:The Null Value, Next:Symbols and Globrefs, Previous:Scalars, Up:Data Conversion

### The Null Value

As an exception to the rule for symbols (see Symbols and Globrefs), `nil` in Lisp corresponds to `undef` in Perl.

In Lisp, `nil` is really a symbol. However, it is typically used as the boolean value false. Perl symbols (glob references) evaluate to true in boolean context. It is very natural to convert `nil` to `undef`.

Node:Symbols and Globrefs, Next:Lists and Arrayrefs, Previous:The Null Value, Up:Data Conversion

### Symbols and Globrefs

Glob references become symbols in Lisp. Underscores are swapped with hyphens in the name, since Perl prefers underscores and Lisp prefers hyphens.

Node:Lists and Arrayrefs, Next:Vectors, Previous:Symbols and Globrefs, Up:Data Conversion

### Lists and Arrayrefs

Lists are a central data structure in Lisp. To make it as easy as possible to pass lists to Lisp functions that require them, Perl array references are converted Lisp lists. For example, the Perl expression

    ["x", ["y", 1]]

is converted to

    '("x" ("y" 1))

in Lisp. Note, however, that the empty list in Lisp is indistinguishable from `nil`, so it becomes the undefined value, not an arrayref. See The Null Value.

Node:Vectors, Next:Conses, Previous:Lists and Arrayrefs, Up:Data Conversion

### Vectors

Adding `\` to an arrayref makes it an arrayref ref, which EPL treats as a vector in Lisp. For example, Perl `\[1, 2, []]` becomes Lisp `[1 2 nil]`.

Node:Conses, Next:Scalar References, Previous:Vectors, Up:Data Conversion

### Conses

Conses that are not lists become Emacs::Lisp::Cons objects.

Compatibility note: In Perlmacs, such conses become opaque objects (see Pass-by-Reference).

    $x = &cons("left", "right");
    print ref($x);                # "Emacs::Lisp::Cons"
    print $x->car;                # "left"
    print $x->cdr;                # "right"

But:

    $x = &cons ("top", undef);    # a Lisp list
    print ref($x);                # "ARRAY"
    print $x->[0];                # "top"

Node:Scalar References, Next:Code References, Previous:Conses, Up:Data Conversion

### Scalar References

Node:Code References, Next:Hash Tables, Previous:Scalar References, Up:Data Conversion

### Code References

Node:Hash Tables, Next:Blessed References, Previous:Code References, Up:Data Conversion

### Hash Tables

The issue with hash tables is that Perl's built-in ones permit only string keys. Right now, hash tables are passed by reference (see Pass-by-Reference).

Node:Blessed References, Next:Emacs Types, Previous:Hash Tables, Up:Data Conversion

### Blessed References

Node:Emacs Types, Previous:Blessed References, Up:Data Conversion

### Emacs Types

Node:Functions, Next:Control Structures, Previous:Data Conversion, Up:Features

## Functions

- Evaluation: `perl-eval`.
- Ordinary Functions: `perl-call`, `use Emacs::Lisp;`.
- Pass-by-Reference: How to inhibit data conversion.
- Commands: Extending Emacs without Lisp, `defun`.

Node:Evaluation, Next:Ordinary Functions, Up:Functions

### Evaluation

perl-eval string &optional context | Function

This function parses and executes string as Perl code and returns its converted value (see Data Conversion). context can specify an evaluation context (see perlsub documentation) and affects how the results are returned. It may be: `scalar-context` Evaluate in scalar context and return the result as a Lisp object. This is the default if context is not given. `list-context` Evaluate in list context and return the result as a list. `void-context` Evaluate in void context and return `nil`.

perl-eval-raw string &optional context | Function

This function executes string like `perl-eval` but does not convert its value. Instead, it returns a `perl-value` object that keeps a reference to a value in Perl.

Node:Ordinary Functions, Next:Pass-by-Reference, Previous:Evaluation, Up:Functions

### Ordinary Functions

perl-call sub &rest arguments | Function

This function calls the Perl subroutine named sub with arguments and returns its value. If the first argument is `scalar-context`, `list-context`, or `void-context`, it is not passed to the sub but affects the calling context as in `perl-eval`. See Evaluation.

Node:Pass-by-Reference, Next:Commands, Previous:Ordinary Functions, Up:Functions

### Pass-by-Reference

XXX Rewrite.

A shallow copy simply wraps a Perl scalar in a Lisp object or vice versa. Wrapped Perl values appear as a Lisp objects of type `perl-value`. Wrapped Lisp values appear in Perl as objects of class `Emacs::Lisp::Object`. See References Incur Overhead, for issues relating to wrapped data.

In Perl, the `wrap` function wraps its argument in a Lisp object. This allows Perl data such as arrays to be passed by reference to Lisp functions. Another way Lisp can obtain references to Perl data is by using `perl-eval-raw` or `perl-call-raw`. Lisp can tell whether an object refers to Perl data by using the `perl-value-p` function.

Of course, `wrap` is a Perl function, so the value it returns is really its argument wrapped in a Lisp object wrapped in a Perl object. But when you pass this object to Lisp as an argument or function return value, Lisp gets a reference to the original object, and no automatic conversion occurs. Lisp may explicitly convert it at any time using the `perl-to-lisp` function, and any changes made by Perl will be visible at the time of conversion.

The Lisp function `perl-wrap` is the counterpart of Perl's `wrap` function, and the `Emacs::Lisp::Object` package provides the equivalent of `perl-call-raw`. The functions of package `Emacs::Lisp` return deep copies of Lisp function return values (see see Functions). However, the corresponding `Emacs::Lisp::Object` functions return references to live Lisp objects.

Since a wrapped Lisp object appears in Perl as an `Emacs::Lisp::Object` blessed reference, if follows that it can be used with method syntax to invoke a function and pass itself as the first argument. Thus, supposing `v` is a vector of conses and `$v` is its wrapper, this Perl code

    $v->aref(3)->setcdr(t);

has the effect of the following Lisp:

    (setcdr (aref v 3) t)

An Emacs::Lisp::Object's `to_perl` method performs a deep copy and is the counterpart of `perl-to-lisp`.

Lisp functions called through package Emacs::Lisp convert their return values using deep copying. The same functions are accessible through Emacs::Lisp::Object, which does shallow conversion and always returns an Emacs::Lisp::Object object.

These examples show how the data wrapping functions work in Perl:

    $x = wrap [1, 2, 3];
    print ref($x);           # "Emacs::Lisp::Object"
    print ref($x->to_perl);  # "ARRAY"
    print @{&list(2, 3)};    # "23"

    $x = Emacs::Lisp::Object::list(2, 3);
    print ref($x);           # "Emacs::Lisp::Object"
    print @{$x->to_perl};    # "23"

And in Lisp:

    XXX

Node:Commands, Previous:Pass-by-Reference, Up:Functions

### Commands

Node:Control Structures, Next:Input/Output, Previous:Functions, Up:Features

## Control Structures

Node:Input/Output, Next:Caveats, Previous:Control Structures, Up:Features

## Input/Output

Node:Caveats, Previous:Input/Output, Up:Features

## Caveats

- References Incur Overhead: Memory management hurdles.
- Conversion Is Imperfect: Natural mappings are not always 1-to-1.

Node:References Incur Overhead, Next:Conversion Is Imperfect, Up:Caveats

### References Incur Overhead

Node:Conversion Is Imperfect, Previous:References Incur Overhead, Up:Caveats

### Conversion Is Imperfect

Node:Configuration, Next:Internals, Previous:Features, Up:Top

# Configuration

- Installation: Lisp files, Perl modules, documentation.
- The Easy Way: We like the defaults.
- Starting a Perl Process in Emacs: `make-perl-interpreter`.
- Starting an Emacs Process in Perl: `use Emacs::Lisp;`.

Node:Installation, Next:The Easy Way, Up:Configuration

## Installation

Node:The Easy Way, Next:Starting a Perl Process in Emacs, Previous:Installation, Up:Configuration

## The Easy Way

Node:Starting a Perl Process in Emacs, Next:Starting an Emacs Process in Perl, Previous:The Easy Way, Up:Configuration

## Starting a Perl Process in Emacs

Node:Starting an Emacs Process in Perl, Previous:Starting a Perl Process in Emacs, Up:Configuration

## Starting an Emacs Process in Perl

Node:Internals, Next:Copying Conditions, Previous:Configuration, Up:Top

# Internals

This chapter introduces the concepts involved in supporting single-threaded distributed exception handling and garbage collection for languages like Perl and Emacs Lisp. These languages have a lot more in common underneath than meets the eye.

The chapter ends with a more concrete description of EPL's protocol. This information changes with great frequency and may be completely outdated by the time you read this.

- Debugging Options: Dumping traffic to stderr or a buffer.
- Transport Layer: Media of interprocess communication.
- Message Types: What interpreters can say to each other.
- State Transitions: The message stream grammar.
- Protocol: Current EPL protocol (subject to change).

Node:Debugging Options, Next:Transport Layer, Up:Internals

## Debugging Options

You can make either or both processes log the messages sent between them.

$Emacs::EPL::debugging | Perl Variable

When this variable holds a true value, the Perl side of EPL logs messages to and from Emacs. Possible values are: `"stderr"` Send debugging output to `STDERR`. This works only when Perl is the parent process. a string Append debugging output to the file named by the string. a scalar reference Append debugging output to the scalar. This requires the IO::Scalar module. a filehandle Print debugging output to the handle.

Emacs::EPL::debug string... | Perl Function

This is the function used internally to send debugging output to the destination specified by `$Emacs::EPL::debugging`.

epl-debugging | Variable

When this variable's value is non-`nil`, the Emacs side of EPL logs messages to and from Perl. If it is the symbol `stderr`, the output goes to the standard error stream. Otherwise, it goes to a buffer named `*epl-debug*`. The log messages include the exact text sent to Perl, but the replies are printed as Lisp forms using `prin1`. However, EPL always uses a buffer named `*perl*` to receive text from Perl. Normally, it erases this buffer after every message, but when `epl-debugging` is true, it does not.

epl-debug &rest objects | Function

This is the function used internally to send output to the location specified by `epl-debugging`.

Node:Transport Layer, Next:Message Types, Previous:Debugging Options, Up:Internals

## Transport Layer

Node:Message Types, Next:State Transitions, Previous:Transport Layer, Up:Internals

## Message Types

Node:State Transitions, Next:Protocol, Previous:Message Types, Up:Internals

## State Transitions

Node:Protocol, Previous:State Transitions, Up:Internals

## Protocol

Node:Copying Conditions, Next:Index, Previous:Internals, Up:Top

# Copying Conditions

You are welcome to distribute EPL under the terms of either the Perl Artistic License (as distributed with Perl 5.6.0) or the GNU General Public License.

> This library is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.

> This software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

> You should have received a copy of the GNU General Public License along with this library; see the file COPYING. If not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

You may distribute copies of this documentation in accordance with the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no required Invariant Sections, Front-Cover texts, or Back-Cover Texts.

- GNU Free Documentation License: The GNU FDL.

Node:GNU Free Documentation License, Up:Copying Conditions

## GNU Free Documentation License

Version 1.1, March 2000

    Copyright © 2000 Free Software Foundation, Inc.
    59 Temple Place, Suite 330, Boston, MA  02111-1307, USA

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other written document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you".

   A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

   The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

   A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not "Transparent" is called "Opaque".

   Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.

   The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five).
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section entitled "History", and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. In any section entitled "Acknowledgments" or "Dedications", preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgments and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section entitled "Endorsements". Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section as "Endorsements" or to conflict in title with any Invariant Section.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

   You may add a section entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgments", and any sections entitled "Dedications". You must delete all sections entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an "aggregate", and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

### ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.1
      or any later version published by the Free Software Foundation;
      with the Invariant Sections being list their titles, with the
      Front-Cover Texts being list, and with the Back-Cover Texts being list.
      A copy of the license is included in the section entitled ``GNU
      Free Documentation License''.

If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being list"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

Node:Index, Previous:Copying Conditions, Up:Top

# Index

- `$Emacs::EPL::debugging`: Debugging Options
- connection types: Transport Layer
- debugging EPL: Debugging Options
- dumping message traffic: Debugging Options
- `Emacs::EPL::debug`: Debugging Options
- `epl-debug`: Debugging Options
- `epl-debugging`: Debugging Options
- FDL, GNU Free Documentation License: GNU Free Documentation License
- Internals: Internals
- interprocess communication: Transport Layer
- IPC: Transport Layer
- Message Types: Message Types
- messages: Internals
- `perl-call`: Ordinary Functions
- `perl-eval`: Evaluation
- `perl-eval-raw`: Evaluation
- pipe between Emacs and Perl: Transport Layer
- protocol: Internals
- protocol states: State Transitions
- standard error stream: Debugging Options
- state diagram: State Transitions
- State Transitions: State Transitions
- stderr: Debugging Options
- Transport Layer: Transport Layer
- wire protocol: Protocol