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MMM Mode is a package for the Emacs text editor/operating system which allows editing files with different sections in several major modes. For example, it can be used to edit Mason components in HTML and Perl modes, switching back and forth based on the position of the cursor. This file is taken from the MMM Mode distribution and gives some pointers on using MMM Mode for Mason.


For general installation and information, see the README file and the texinfo documentation. The submode class for Mason components is called `mason' and is automatically loaded from `mmm-mason.el' the first time it is used.


If you want to have mason submodes automatically in all Mason files, you can use `mmm-mode-ext-classes-alist'; the details depend on what you call your Mason components and what major mode you use. Some example elements of `mmm-mode-ext-classes-alist' follow, with comments on the corresponding naming scheme.

(html-mode "\\.html\\'" mason) ;; Any .html file in html-mode
(hm--html-mode nil mason) ;; Any buffer in hm--html-mode
(sgml-mode nil mason) ;; Any buffer in sgml-mode
(nil "\\.\\(mason\\|html\\)\\'" mason) ;; All .mason and .html files
(nil "\\.m[dc]\\'" mason) ;; All .md and .mc files
(nil "\\`/var/www/mason/" mason) ;; Any file in the directory
(nil nil mason) ;; All buffers.

In order for any of these to work, you must set `mmm-global-mode' to a non-nil value, such as `t' or `maybe' (the two of which mean different things; see the documentation). This can be done with a line in .emacs such as the following:

(setq mmm-global-mode 'maybe)

If you use an extension for your Mason files that emacs does not automatically place in your preferred HTML Mode (be it html-mode, sgml-html-mode, hm--html-mode, or whatever), you will probably want to associate that extension with your HTML Mode (this is a feature of emacs, not MMM Mode). An example is shown below.

(add-to-list 'auto-mode-alist '("\\.mason\\'" . html-mode))

This also goes for "special" Mason files such as autohandlers and dhandlers. The code below tells emacs to use html-mode for files named `autohandler' and `dhandler'.

(add-to-list 'auto-mode-alist '("\\(auto\\|d\\)handler\\'" . html-mode))

An alternate solution is to change the names of your autohandlers and dhandlers so that emacs recognizes them as HTML automatically. Similar code can be used to recognize all files in a given directory as HTML and/or Mason.


There are certain problems with CPerl mode in submode regions. (Not to say that the original perl-mode would do any better--it hasn't been much tried.) First of all, the first line of a Perl section is usually indented as if it were a continuation line. A fix for this is to start with a semicolon on the first line. The insertion key commands do this whenever the Mason syntax allows it.

print $var;

In addition, some users have reported that the CPerl indentation sometimes does not work. This problem has not yet been tracked down, however, and more data about when it happens would be helpful.

If you install CPerl 5.0 (the latest, as of this writing) you're likely to loose Perl coloring in inline regions. Indentation problems, however, should disappear.


Some people have reported problems using PSGML with Mason. Adding the following line to a .emacs file should suffice to turn PSGML off and cause emacs to use a simpler HTML mode:

(autoload 'html-mode "sgml-mode" "HTML Mode" t)

Earlier versions of PSGML may require instead the following fix:

(delete '("\\.html$" . sgml-html-mode) auto-mode-alist)
(delete '("\\.shtml$" . sgml-html-mode) auto-mode-alist)

Other users report using PSGML with Mason and MMM Mode without difficulty. If you don't have problems and want to use PSGML, you may need to replace `html-mode' in the suggested code with `sgml-html-mode'. (Depending on your version of PSGML, this may not be necessary.) Similarly, if you are using XEmacs and want to use the alternate HTML mode `hm--html-mode', replace `html-mode' with that symbol.

One problem that crops up when using PSGML with Mason is that even ignoring the special tags and Perl code (which, as I've said, haven't caused me any problems), Mason components often are not a complete SGML document. For instance, my autohandlers often say

<% $m->call_next %>

in which case the actual components contain no doctype declaration, <html>, <head>, or <body>, confusing PSGML. One solution I've found is to use the variable `sgml-parent-document' in such incomplete components; try, for example, these lines at the end of a component.

%# Local Variables:
%# sgml-parent-document: ("autohandler" "body" nil ("body"))
%# sgml-doctype: "/top/level/autohandler"
%# End:

This tells PSGML that the current file is a sub-document of the file `autohandler' and is included inside a <body> tag, thus alleviating its confusion, and also instructs it where to find the doctype declaration (assuming your top-level autohandler has one). This alleviates most problems for me. I admit to not understanding PSGML internals very well, so YMMV.

-- MichaelShulman

Unfortunately, syntax highlighting with MMM-mode is seriously broken in GNU Emacs 21.x There have been some changes in font-lock.el that prevented the highlighting of inline regions, and lines starting with %.

To work around this, use font-lock.el from Emacs 20.x. There are also problems with <% %> and <& &> sometimes interfering with quotes in html-mode. Using html-helper-mode instead of html-mode fixes this.
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I like the MM mode package alot.