PLEXIL, XML, and Emacs

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Core PLEXIL, the executable form of PLEXIL, is specified in the Extensible Markup Language, or XML. This XML representation of PLEXIL is hidden from the user in general use. This chapter provides information about PLEXIL's XML definition, and tools for inspecting and editing Plexil XML files, if the need should arise.

Plexil may be coded in any text editor. The Emacs editor in particular has features suitable for PLEXIL, namely its powerful XML editing and validation capabilities, as well as being the platform for the alternative PLEXIL syntax Plexilisp.

This chapter describes helpful XML and Emacs resources for PLEXIL.


An XML Schema is a file that defines the format for a particular kind of XML file. PLEXIL XML is defined by schema files found in the PLEXIL distribution, under plexil/schema. See the README file in that directory for detailed information.

Schema Validation

An XML file may be validated against an XML schema. There are many tools for this. Both Linux and Mac OS provide xmllint, a Unix command line schema validator. Several commercial graphical tools, such as Oxygen are also available. Schema validation can also be done interactively using Emacs; this is described below.


For decades Emacs has been a popular text editor for programming. It is a featureful and highly customizable editor and integrated work environment, available for every common computer platform.

This document does not provide detailed instructions for Emacs, and largely assumes the reader to be already fairly comfortable with Emacs. If you are just starting out with Emacs, or interested, please consult the abundance of introductory material available on the Web. Emacs usually comes standard on Unix-based systems such as Linux and Mac OS, however, there are variants of Emacs available that are more featureful than these built-in versions.

Emacs is helpful for viewing, validating, and, if desired, editing Core PLEXIL XML. Most versions of Emacs have some built-in support for editing XML files. When you open or create a file with the extension .xml, the resulting buffer will usually be in an XML-specific editing mode.

Emacs Setup

When Emacs starts, it loads the file ~/.emacs, which is where user customizations are generally placed. The following code will make Emacs more Plexil-friendly. You'll need to customize the pathnames for your environment.


;;; Set the required PLEXIL_HOME environment variable with Emacs. (Although it ;;; may already be set in your Unix environment, Emacs does not always read this ;;; setting.
(setenv "PLEXIL_HOME" "/users/fred/plexil")

;;; Edit .plx files as XML.
  (setq auto-mode-alist
    (cons '("\\(plx\\)\\'" . xml-mode) auto-mode-alist))

NOTE: if you install nXML (described below), you'll want to substitute nxml-mode for xml-mode.

Schema Validation Using nXML Mode

It is possible to validate XML files against a schema within Emacs, using a freely available Emacs utility for XML called nxml mode. It provides not only an easy means to edit XML (e.g. automatic indentation), but also a means for validating your XML against an XML schema interactively.

Getting nXML

First, check if your Emacs already has nXML. Open or create an XML file and see if the mode line (text bar at bottom of the buffer) contains "(nXML ...)". If not, see if the following command is available:

M-x nxml-mode

If not, you'll need to download nXML. Information can be found at the following web sites.

Installing nXML

To automate the loading of nXML and ease its use, the following code can be added to your .emacs file. Edit the file path as needed for your system.

;;; Load NXML mode
 (load "/Applications/nxml-mode-20041004/rng-auto.el")

 ;;; Make standard XML file extensions engage nxml-mode
 (setq auto-mode-alist
 (cons '("\\(xml\\|xsd\\|xsl\\|rng\\|xhtml\\)\\'" . nxml-mode)

 ;;; Make nxml-mode override Emacs's native xml-mode
 (push '("\\`<\\?xml" . nxml-mode) magic-mode-alist)

Using nXML

Once installed, any XML file you edit in Emacs will be done so in nXML mode.

To validate against a schema using nXML, you'll need to have a schema in Relaxed Compact NG (RNC) format. The PLEXIL schemas are provided in this format. The "XML" menu in Emacs provides various methods of validation, one of which requires you to specify a schema file. However, to have any XML file in a given directory be automatically validated by the same schema file, create a file called schemas.xml in this directory, with the following contents (change the schema file pathname as needed).

<?xml version="1.0"?>
<locatingRules xmlns="">

 <default uri="../../schema/extended-plexil.rnc"/>


There is more information about nXML in plexil/schema/Makefile.

Copyright (c) 2006-2014, Universities Space Research Association (USRA). All rights reserved.