Buffer Menu Sorting in Emacs 24

Upgrading to Emacs 24 a few months ago was mostly seamless. I only encountered one issue that was difficult to fix and has been lingering until today, when I finally got annoyed enough to devote some time to researching and fixing it.

I like having the Buffer Menu configured to always be sorted by filename. In Emacs 23, you could simply set the Buffer-menu-sort-column variable and you were done. That variable isn’t recognized in Emacs 24, despite claims to the contrary on various web sources.

If you look at the code in buff-menu.el, it was rewritten in 24 to use tabulated-list for its display mechanics. There is a Buffer-menu-sort function which is an alias to tabulated-list-sort. Once you are in the *Buffer List* buffer, you can call Buffer-menu-sort interactively using a prefix argument to specify the column: type ‘M-5 M-x Buffer-menu-sort’ to sort by filename. This works very nicely. Doing it a second time reverses the sort order.

So the challenge is to automate this, calling Buffer-menu-sort only once, automatically at the time a *Buffer List* buffer is first created. The mode will keep its entries sorted going forward.

I tried doing this by advising the list-buffer function, and having a buffer local variable keep track of whether I had already called Buffer-menu-sort once before. This didn’t work: something seemed to be resetting my buffer local variable, but I couldn’t figure out what.

The solution I ended up with is below. The advice function checks a variable called jc-buffer-menu which stores the current Buffer Menu buffer, and calls Buffer-menu-sort as needed. Put the code in your .emacs file, and you’re done.


string-match and dealing with state

(or, How I Spent a Few Hours Wrestling with Emacs)

I encountered some strange behavior and errors when calling replace-regexp-in-string in some Emacs code today.

My code was passing a function as the REP argument to replace-regexp-in-string. Here’s a similar, simplified example:

Fine, that’s what we expected. Now let’s change how mask-ssn works. It should still do the same thing, but does it?

Whoa, that’s not right at all!

What happened? Well, there’s a while loop in replace-regexp-in-string that calls string-match, then calls the REP function, and then uses the result in a call to replace-match. Since mask-ssn also calls string-match (via split-string), this messes up the next call to replace-match in the loop.

Figuring this out took a while, as my actual function was way more complicated than mask-ssn. I didn’t even suspect string-match to be the problem initially, so I went through the process of taking everything out and adding code back in line by line, chunk by chunk, call by call, until I pinned it down. In addition to weird string replacements, I also sometimes got an “args-out-of-range” error, depending on the size of the strings and the various functions using string-match that I experimented with calling.

There IS a solution, happily: save-match-data, which is better documented here than in the Emacs help. In a nutshell, the macro saves the current match state, evaluates the forms inside the body, and then restores the state afterwards.

That does the trick.

I discovered this only after going through the trouble of building a replacement to replace-regexp-in-string.

I felt pretty happy with myself after writing that. Since it doesn’t call any match functions after calling REP, all is well.

But then I thought about it some more. my-replace ITSELF is changing match data, so any callers will suffer from the same problem. D’OH. So we still need to wrap our code in save-match-data, if we’re going to be responsible.

I think save-match-data illustrates an interesting “pattern” of sorts. It deals with a problem of state, which is unavoidable, as we’re concerned with needing to track searches of text buffers. Since we HAVE to store this state, we can’t do it in a clean, purely functional way.

save-match-data handles this by saving the state of the string match in progress, probably pushing it on a stack somewhere (I didn’t dive too deep), letting the wrapped code manipulate the world, and then popping the original state off the stack to restore it afterwards. From the outside, it seems as if no state has changed, between the point at which you enter save-match-data and the point at which you exit. This allows for nested code, any number of call levels deep, to do what it needs, without interfering with other levels, as long as the relevant chunks of code are wrapped in the macro.

Pretty cool.

Making Emacs an IDE

It’s that time when bloggers wax introspective about the past year. For me, the major personal revelation in 2011 was re-discovering something very old, and putting it to new use. For me, 2011 was the year of the Emacs IDE.

I’ve been using Emacs, on and off, for close to a decade now. What’s changed is that, in the past few months, I’ve been writing extensions for it. It started with a simple desire to better navigate files in a complex directory hierarchy that followed specific and somewhat convoluted conventions. At first, learning Emacs Lisp was simply a means to an end, but I ended up liking it so much that I started exploring Common Lisp (and more recently, Clojure, since I’ve worked with Java in the past).

What started as a small task has become a larger project of turning Emacs into an IDE.

To understand this, one needs to know some context about the system I work with. We developers edit tons of XML files and files in other text formats, which all drive our proprietary web application product. We have many command line tools that manipulate these files in various ways; the system was originally designed by folks who followed the UNIX philosophy of building orthogonal tools and chaining them together.

There are pros and cons to this system; for reasons I won’t get into, I don’t love it, but it’s what we work with right now. When I started the job, the vast majority of the developers used screen, vi, and the shell prompt. Typical workflows that involved working with only a few files could be extremely hard to keep track of, and usually required a lot of copying and pasting between screen and/or ssh sessions. Few people seemed to mind, but I found the workflow to contain too much extraneous cognitive load, and the state of the tools made development very prone to error.

Gradually, I’ve been integrating our tools into Emacs. Sometimes that simply means binding a key combination to running a diagnostics program and storing the output in a buffer. Sometimes it means accumulating that output for history’s sake. Sometimes it means parsing program output, processing it in Emacs Lisp, and inserting a result into the current buffer. Sometimes it means running external programs, even GUI applications, and tweaking them a bit to tell Emacs to open specific files you want to look at.

The productivity gains have been amazing. This is no reason to brag: managing several screen sessions with different vi and shell instances wasn’t exactly hard to improve upon. But Emacs made it fairly painless. Emacs Lisp has proved to be wonderful “glue” for integrating existing tools in our environment.

Writing tools that enable you to do other job tasks better is a really interesting experience; I’ve never done it to such an extensive degree. So far, one other person in my group is using this Emacs IDE, and she has been happy with how much it facilitates her work. Others who swing by my desk for something often watch me work for a moment, and ask, “how did you do that?! that’s not vi, is it?”

Getting more people to switch over means convincing them that the steep learning curve of Emacs is worth the gains of the integration I’ve done. I’m not sure how much that will happen, since a big part of it is cultural. But if there aren’t any more converts, I don’t really care. The best thing about this ongoing project is that I am the end user. The software I wrote is really for myself. It is something I use intensively every single day. And that makes it all the more gratifying.

Getting the Solarized Theme to Work in Emacs

I discovered the Solarized color palette about a month ago, and have been using it since. I’ve found it helpful for reducing eye strain.

Getting the palette colors to accurately display in terminal-based Emacs under Ubuntu 11.10 took a bit of work. There’s a lot of scattered info about this topic if you google around. I first tried to do it with as little modification to my system as possible (I’m a minimalist), but I found that I absolutely had to do these three things to get it working.

1) Set the color palette in gnome-terminal. No matter what I tried, I could not get Emacs colors to look right without this step, so it’s a crucial one. Don’t skip it.

Solarized comes with an Xdefaults file, but that didn’t work for me. This blog post had instructions for setting gnome-terminal colors using gconftool-2, and that worked very nicely.

I put the commands in a handy little shell script that allows you to choose either the light or dark version of the palette. You can get it here:


2) Your TERM environment variable needs to be set to “xterm-256color.” The default “xterm” will cause the Solarized color theme to look completely crazy in emacs.

Add the following line to your ~/.bashrc line (if you already set TERM there or some elsewhere else, change it instead of adding this):

export TERM="xterm-256color"

3) Use this version of the Emacs color theme for Solarized:


Note that unless you’re using Emacs 24 (pre-release source code tarballs are available here), you’ll also need the color-theme package if you don’t already have it installed.

Put the files in a directory. In your .emacs file, you’ll need to add the directory to your load-path; I also added a bit of conditional logic for how to load the color theme in Emacs 23 and 24, since I switch between them:

(add-to-list 'load-path "~/.emacs.d/emacs-color-theme-solarized")
    (equal 0 (string-match "^24" emacs-version))
    ;; it's emacs24, so use built-in theme 
    (require 'solarized-dark-theme)
  ;; it's NOT emacs24, so use color-theme
    (require 'color-theme)
    (require 'color-theme-solarized)

That should do it.

Learning Lisp

For work projects, I often have to jump around a lot of XML files referenced in one another by name. The files follow a strict naming convention, but it’s one that looks like gibberish to human eyes. Typing them is difficult and prone to error; plus there are so many files in a given project that navigating a visual tree of the directory isn’t much easier.

So I wrote a small library for Emacs to find and open a file in the directory tree, if the cursor is over a string of text that matches a valid filename in the convention. I’m embarrassed to admit this took me two full days (a few other functions were in the library too). The only previous exposure to Emacs Lisp I had was tweaking my .emacs file, so I had to learn the language as well as hunt down the right functions to call for what I wanted. The work is paying off: it feels 10x’s easier to navigate project files now.

I experimented with trying to do this in Komodo Edit, a popular editor among coworkers, but it involved learning Mozilla’s XUL and Komodo’s own API, as well as writing javascript (yuck), so I abandoned that effort pretty quickly.

I didn’t think I would like Lisp, but the experience has been pretty fascinating, and I’m now making my way through the Practical Common Lisp book by Peter Seibel. It’s interesting learning a language that’s half a century old (!), and that’s influenced so many contemporary languages. One might think that there’s nothing there worth learning or revisiting, but that is so wrong. In particular, I’m trying to wrap my head around the power of Lisp macros and the way they allow you to create new syntactic abstractions. The idea of extending the very language itself, rather just adding new functions, is mind-boggling, to say the least. And, from what I understand, it remains fairly unique to Lisp in spite of the flood of new languages in the past few decades.

It’s disheartening not being able to find much info about who actually uses Lisp anymore, aside from hackers building modules for Emacs. Paul Graham has a cool essay, “Beating the Averages”, about using Lisp to build online store software that Yahoo eventually acquired. And ITA software, which makes an airfare search engine that powers the entire travel industry, uses Lisp. But aside from these bits of info, there isn’t much out there in the way of Lisp “success stories.”