What I cannot create, I do not understand.

Just because you've implemented something, doesn't mean you understand it.

Setting up Emacs for Lisp hacking on OS X, pt. 1: Scheme

This weekend I took a bus from NYC to Boston and then on to Portland, ME for the holiday (it’s Thanksgiving weekend here in the US). I find it hard to do any real work on a bus because I tend to get a little car sick (I was quite a puke-y kid growing up) so I decided to start fiddling with my Emacs set up, which wouldn’t take too much deep thought.

I’m going to be posting my notes over the next few days; they might be useful for someone else. I’ve benefited greatly from people posting howtos like this on random blogs, plus when I hose it I can always refer back to this.

Install some implementation of Scheme if you don’t have one installed already. I’ve been using MIT Scheme for SICP, but you can get Gambit Scheme, Scheme 48 and PLT Scheme (via mzscheme) through Homebrew. MIT Scheme has a pre-built binary for OS X which includes their Edwin Emacs clone and a command line interpreter.

Vanilla Emacs is also in Homebrew, and probably in Macports and Fink as well. For Homebrew, you’ll probably want to specify --cocoa when installing it to get Emacs.app, which I’d advise you move into the OS X /Applications directory, which will allow you to start Emacs via Spotlight.

I would highly recommend checking out Phil Hagelberg’s “Emacs Starter Kit” which includes a huge number of customizations and niceties for getting started with Emacs. The best way to use this is to fork the project on Github, create your own branch (or not), clone it on your machine and then move the directory to ~/.emacs.d, being careful if you already had some Emacs stuff in there you want to keep.

He provides a way of adding your own customizations by creating a ~/.emacs.d/username.el where username is replaced by your system username. Now you can add all your own stuff in there, or add stuff to override things you don’t want from the Emacs Starter Kit (personally I don’t like the faded parens, visual bell, and pretty lambdas, but that’s all largely personal preference). Now you can keep your changes separate and pull updates the the starter kit in the future, without messing everything up.

To get a Scheme REPL inside of Emacs, you need to configure Emacs to use the particular Scheme binary of our implementation. Since I’m using MIT Scheme, this means /Applications/mit-scheme.app/Contents/Resources/mit-scheme. If you are using something else, use the path to that binary instead. I tried it with mzscheme from Homebrew and it worked, but I don’t know about the others.

Add this to your ~/.emacs.d/username.el, replacing string with the path to your Scheme binary:

(setq scheme-program-name 

MIT Scheme has an extended interface for interacting with the REPL called “xscheme”, which includes things like M-o for evaluating an entire buffer. This should be included with Emacs. (If it’s not, you can get it here )

Add this to your ~/.emacs.d/username.el file to use this interface:

(defun load-xscheme () (require 'xscheme)) 
(add-hook 'scheme-mode-hook 'load-xscheme)

Now M-x run-scheme will open your chosen Scheme implementation in a new buffer and let you send the expression before the cursor with C-x C-e.

If the extra xscheme commands don’t work, or if Emacs throws up when you start it, maybe there is some issue with the xscheme.el library. Try downloading it separately, saving it into the ~/.emacs.d directory and adding this to your ~/.emacs.d/username.el file:

(load "~/.emacs.d/xscheme.el")

Restart Emacs and try the xscheme commands again.

I’ll add part two, for setting up Common Lisp and Clojure using SLIME, sometime soon.