1. Overview

As you are probably aware, Emacs does various sorts of completion, if you ask it to. For instance, if you hit C-x C-f to find a file, then (once you are in the minibuffer) if you hit the TAB key, Emacs completes the file name if there is a unique completion. Lightning completion offers an improvement to this, an improvement in several ways:

• Once lightning completion is initiated, it remains active until a completion is found, or until turned off. If I have three files, `alphabet', `alpo', and `bozo', and I initiate completion on file names, if I hit a, then `alp' is inserted; hitting h would then produce `alphabet', and hitting o would produce `alpo'. Similarly, hitting b at the start, instead of a, would have produced `bozo'. This sort of completion, in which completion is always on--you don't have to keep hitting the SPC or TAB key to complete--is called "dynamic" completion.

• Lightning completion takes effect only if Emacs is idle for a length of time specified by the variable lc-complete-idle-time-default, the default value of which is 0.5 seconds. So in the above example, you can type "alph" or "alpha" or "alphab"; then if you wait for half a second, Emacs will automatically complete it for you. This is a new feature of lightning completion; if you are used to the old version, you can set this delay to be 0. If you are a slow typist and you don't want Emacs completing unless you take a very deliberate pause, you can set it to something like 2.

• Lightning completion can be initiated anywhere, not just in the minibuffer. If I'm writing email to someone, and I want to include the path name for some file, I type the first few letters of the path, then run the function completing-insert-file-name, and I'm in lightning completion mode. There are similar function calls for completion on buffer names, lisp objects, lisp functions, lisp variables, and the contents of the kill ring.

Some remarks:

1. Dynamic completion, like any completion, has a nice side-effect: you can use long (and hence descriptive) file names, variable names, function names, whatever, because you hardly ever have to type the whole thing in. So I don't mind that my directory structure in my account is deep and the directories have long names--I can find the file `~/tex/work/papers/hovey-strickland/axiomatic.tex' with 6 keystrokes: one each for "t", "w", "p", "h", and "a", and then RET. (One also gets in the habit of trying to arrange it so that each of one's files in any given directory starts with a different letter.)

2. If you mainly use lightning completion with file names and if you edit the same files over and over again, you will quickly learn exactly which characters you have to type to find each file. In this case, you might want to set lc-complete-idle-time-default to be close to zero. If you don't want to bother learning this, and if you are worried that lightning completion will make you watch the screen all the time to see what has been completed and what hasn't, then you should set lc-complete-idle-time-default to be a larger number, so that completions only occur after a good pause.

3. Lightning completion is also one of the main ingredients to Ultra-TeX mode, a major mode for typing TeX documents. Most TeX commands start with a backslash, so in that mode, hitting "\" starts lightning completion on TeX commands. The Ultra-TeX package is available from http://www.math.washington.edu/~palmieri/Emacs/ultratex.html.

4. This package should be compatible with GNU Emacs 19, GNU Emacs 20, and XEmacs ("should be compatible" means that I've at least tried it out a bit with each one of these, without running into problems). I mostly use GNU Emacs 20, so I know it works there. I expect it to work pretty well with XEmacs, too, but I don't use GNU Emacs 19 anymore, and I don't test this package very vigorously there.

5. Dynamic completion is an acquired taste. It can be rather off-putting at the start, especially with a short lc-complete-idle-time-default, but once you get used to it, it's just wonderful.

In the following sections, I describe what functions to call to start lightning completion, which keys do what once completion is engaged, how to automate things a bit (e.g., so that find-file automatically uses lightning completion on file names), and how to install all of this.