Editing GAP files and running GAP in Emacs buffers


(Written 20 Feb 1993) (Revised 1 Nov 2011)

The files "gap-mode.el" and "gap-process.el" provide modes for both editing GAP programs in Emacs and running a GAP session within an Emacs buffer. Brief installation instructions are given at the end of this document.

The latest version can be found at .

Editing GAP files in Emacs

Opening any file ending in ".g" or ".gap" should automatically put you into gap-mode, the major mode for editing of GAP code. This mode may also be invoked in any buffer at any time by typing M-x gap-mode.

Once in gap-mode there are some notable changes in the behaviour of Emacs. Whenever you press return for a new line Emacs will reindent the current line and auto-indent the new line (this behaviour can be deactivated). At any time, the TAB key will reindent the current line, M-q will reindent each line in the current region, and M-C-q will reindent each line in the whole buffer.

Gap-mode will add indentation for if..then structures, function definitions and all looping structures,

for N in [1..10] do
    Print ( N );
    if N > 5 then
        Print ( N^2 );
od; ,

as well as indenting continued statements (those that cross a line break) in a number of different ways. For example, it will attempt to match up each line of a matrix,

x := [ [ 1, 2, 3 ],
       [ 4, 5, 6 ],
       [ 7, 8, 9 ] ];

and the arguments of a function call,

Print ( a, b, c
        d, e, f ); .

There are quite a number of variables that control how gap-mode indentation behaves. Consult the help for gap-mode by typing C-h m for a list of the variables (and the features of gap-mode in general), and then C-h v <var> for a description of what the variable <var> controls. It is also suggested that you read the GAP menu in emacs to find the commands which might be useful.

Running GAP in an Emacs buffer

Type M-x gap to run a GAP process with input and output through an Emacs buffer. Any text typed at the end of the gap buffer will be sent to GAP when the RETURN key is pressed, and GAP's output will be appended to the end of the buffer. The mode is based on comint-mode.

Moving back through previous commands is slightly different. Use M-p and M-n for previous and next input. The command M-l will find the last input that matches what has already been typed. There are some other features that are inherited (as these are) by using comint-mode as a base (see the documentation for gap-process-mode by typing C-h m in the gap buffer, and also the help for comint-mode: C-h f comint-mode).

TAB will complete as usual, except that if there is no unique (partial) completion then the list of completions will be given immediately in a separate Completions buffer. Similarly the help function ?, which will ask for a topic (defaulting to the current identifier), will give its results in a GAP Help buffer instead of the gap buffer.

In fact, if a GAP process is running in the gap buffer and NOT BUSY with a calculation, then completion and help are also available in the gap editing mode (gap-mode) by typing M-TAB and M-? respectively.

When starting up the GAP process, giving a prefix argument to the command (eg by typing C-u M-x gap) will cause the contents of the current buffer to be given to GAP as initial input, and GAP will behave exactly as if you had typed all the current buffer contents into the new gap buffer. You can also send the contents of the current buffer to the gap buffer later via C-c C-b, or if the buffer is backed by a file by C-c C-f which sends Read statement and therefore is faster. You can also send the current function defition, current statement, or current region.

There is support for adding local variable statements to functions. Typing C-c l while in a function definition will add (or regenerate) a statement with all the local variables. Unfortunately, it will incorrectly view global variables as being local if they are assigned to. Typing C-c a will add the current identifier to a preexisting local variable list (or creates a new one if gap-local-variable-inserts-statement is non-nil).



You can now install gap-mode through MELPA ( This makes installation easier and especially makes it easier to update. If you have Emacs 24 (or later) then package.el is already included. Otherwise you will have to install it by downloading an old version from

After than you have to tell it to use MELPA, so add something like

(setq package-archives
      '(("melpa"     . "")
        ("gnu"       . "")))

to your .emacs. Finally, run package-list-packages, find gap-mode, type i (for install) and then x (to execute). It will download the latest version and install it automatically. Running package-list-packages in the future will allow you to easily updage gap-mode.

Manual Installation

Put the files "gap-mode.el" and "gap-process.el" into a directory in your Emacs lisp load path, and add the following lines to your ".emacs" startup file.

;; gap mode
(autoload 'gap-mode "gap-mode" "Gap editing mode" t)
(setq auto-mode-alist (append (list '("\\.g\\'" . gap-mode)
                                    '("\\.gap\\'" . gap-mode))
(autoload 'gap "gap-process" "Run GAP in emacs buffer" t)


For gap-mode to function properly you will have to set a few variables, either via M-x customize-group RET gap RET, or by setting them directly in your .emacs like

(setq gap-executable "/usr/algebra/bin/gap")
(setq gap-start-options (list "-l" "/usr/algebra/gap3.1/lib"
                              "-m" "2m"))

If you like to see the help inside emacs, but use other settings inside a terminal then you can add the following to your .gaprc file:

# Versions 4.4 and 4.5 use different formats for environment variables
if (IsRecord(GAPInfo.SystemEnvironment) and
    IsBound(GAPInfo.SystemEnvironment.INSIDE_EMACS )) or
   (not IsRecord(GAPInfo.SystemEnvironment) and
            x->Length(x) > 13 and x{[1..13]} = "INSIDE_EMACS=")) then

    # Emacs colors the prompt anyway
    ColorPrompt( false );
    # See help in emacs
    # Print all the help without paging -- faster and more robust
    PAGER := "tail";
    PAGER_OPTIONS := [ "-n" ];

    # Whatever pager etc. you like in a terminal


Originally written by

Michael Smith Mathematics Research Section Australian National University.

Contributions by Gary Zablackis and Goetz Pfeiffer

Now maintained by

Ivan Andrus