This is edition 0.3pre of the EPL Reference Manual, corresponding to EPL version 0.8pre.
EPL (Emacs Perl) is a library of Lisp functions and Perl modules that let you write Emacs extensions in Perl.
Appendices
--- The Detailed Node Listing ---
Introduction to EPL
&function().
How Language Features Correspond
throw, signal, die.
Configuration
make-perl-interpreter.
use Emacs;.
Internals
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.
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)).
&function().
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.
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.
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.
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.
defun.
throw, signal, die.
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.
undef and () are nil.
\*::sym is 'sym.
["fnord"] is '("fnord").
\[42] is [42].
Emacs::Lisp::Cons type.
perl-ref type.
\&frob is a lambda expression.
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 + 0.0), 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.
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.
Glob references become symbols in Lisp. Underscores are swapped with hyphens in the name, since Perl prefers underscores and Lisp prefers hyphens.
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.
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].
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"
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).
perl-eval.
perl-call, use Emacs::Lisp;.
defun.
| 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:
|
| 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.
|
| 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.
|
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
make-perl-interpreter.
use Emacs::Lisp;.
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.
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:
|
| Emacs::EPL::debug string... | Perl Function |
This is the function used internally to send debugging output to the
destination specified by $Emacs::EPL::debugging.
|
| epl-debugging | Variable |
When this variable's value is non-nil, the Emacs side of EPL
logs messages to and from Perl. If it is the symbol stderr,
the output goes to the standard error stream. Otherwise, it goes to a
buffer named *epl-debug*.
The log messages include the exact text sent to Perl, but the replies
are printed as Lisp forms using |
| epl-debug &rest objects | Function |
This is the function used internally to send output to the location
specified by epl-debugging.
|
You are welcome to distribute EPL under the terms of either the Perl Artistic License (as distributed with Perl 5.6.0) or the GNU General Public License.
This library is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.This software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this library; see the file COPYING. If not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
You may distribute copies of this documentation in accordance with the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no required Invariant Sections, Front-Cover texts, or Back-Cover Texts.
Version 1.1, March 2000
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$Emacs::EPL::debugging: Debugging Options
Emacs::EPL::debug: Debugging Options
epl-debug: Debugging Options
epl-debugging: Debugging Options
perl-call: Ordinary Functions
perl-eval: Evaluation
perl-eval-raw: Evaluation