Common Lisp Programming and emacs

810:161

Artificial Intelligence Laboratory

Editor's Note

This document was created by Sid Bos, a former UNI student of AI, in response to questions from several of his fellow students about how to use emacs to support Common Lisp programming. I thank Sid for the effort he put into to writing the original document. I have since made a number of modifications. I have re-formatted the document for use in 810:161, have made small changes to content and grammar, and this semester have modified it to work in our new Linux environment and CMU Common Lisp. Any errors should be construed as mine and reported to me for correction. -- EW

Introduction

Gnu emacs can provide an interface to any Common Lisp system. On cowboy, it provides an interface to lisp, the CMU Common Lisp system.

If you would like to use this interface but are a little rusty on how to use emacs, be sure to review the next section before proceeding. If you are pretty comfortable with your user level in emacs, feel free to proceed directly to the Common Lisp portion of this page. If you want to see all of the commands discussed below in the context of a Common Lisp/editing session, jump directly to a short laundry list.

A Quick Refresher on emacs

emacs uses three types of control sequences to specify commands:

• M-x means to press and release the Escape key, followed by the character x. (The Escape is pronounced "meta".)

• C-x means to press both the Control key and the character x at the same time.

• C-c e means to press both the Control key and the character c, and then to press the character e.

You will want to know the following commands in order to navigate as a power user within emacs:

• C-x 2 - Split the screen into two windows (called buffers).
• C-x 1 - Make the current buffer the only buffer shown.
• C-x o - Move to the other buffer on the screen.
• C-k - Kill the current line. Pressing again kills the carriage return as well.
• C-y - Yank whatever was last killed.
• C-g - This will cancel the current operation, for which Emacs is asking for input.
• C-x C-b - This will give you a list of the current buffers you have open.
• M-v - Move up one page in the buffer.
• C-v - Move down one page in the buffer.
• C-a - Move to the beginning of the current line.
• C-e - Move to the end of the current line.

Supporting Common Lisp within emacs

emacs provides different modes that are helpful when programming in different languages. They offer support for programming style (e.g.,automatic indentation) and syntax (e.g., parenthesis balancing). emacs' current mode should be displayed in the status bar at the bottom of its window.

In order to take advantage of built-in support for Common Lisp programming, you will need to be in lisp-mode. To switch to lisp-mode, type M-x lisp-mode. If you want emacs to enter lisp-mode automatically for files with a .lisp extension, add the following expression to your ~/.emacs file:

    (setq auto-mode-alist (append '(("\\.lisp$" . lisp-mode))
                                  auto-mode-alist))

Yes, this is Common Lisp code. Gnu emacs is written almost entirely in Common Lisp!

Gnu emacs actually provides its own built-in Common Lisp interpreter, which you can run by typing

Unfortunately, this version of Common Lisp is so incomplete that it won't be sufficient for programming in our class.

Instead, you can run and use lisp within emacs. Here are the relevant commands:

• M-x inferior-lisp - Starts CMU Common Lisp in the current buffer window.

• C-c C-e - When you are in a buffer that is in lisp-mode and you have a buffer running inferior-lisp, this will evaulate the current Common Lisp expression -- the one the cursor is in. If the cursor is in a function definition, lisp will reload the the function definition from the emacs buffer, even if you have not yet saved the file.

The following commands can be used within the buffer in which lisp is running. If you are at the command line within the lisp buffer,

• M-p - Pulls up the previous command.
• M-n - Pulls up the next command.

I know that many of you have been clamoring for this capability!

A Common Lisp/Editing Session

Here is what you might do when you sit down at the machine with the intention to work on Common Lisp code.

• Have the auto-mode line in your .emacs file, so that all *.cl files come up in lisp-mode.

• At the Unix prompt, type emacs <filename>.cl, where <filename>.cl is the Common Lisp file that I want to edit.

• When the emacs window comes up, type C-x 2 to pull up two buffer windows.

• Type C-x o to switch to the other window.

• Start lisp by typing M-x inferior-lisp.

• Type (load "<filename>.cl") to load the file into the Common Lisp buffer.

• Type C-x o to move to the buffer where your Common Lisp code is.

• Edit the file as necessary.

• Type C-c C-e to load the modified function definition into the gcl interpreter.

• Type C-x o to move back to the gcl buffer.

• Type M-p to bring back the previous command.

When you're done, be sure to save your file to preserve the results of the session. (Even though the inferior gcl session loads directly from the unsaved buffer, you will want to save for future use!)