Tuesday, January 7, 2014

Simple scripting in Common Lisp

I know enough Bash to read simple scripts, but I am always having a hard time when I am trying to write a script that does something that is more complicated than a one-liner that substitutes arguments into a template. I can eventually figure things out, but I have never bothered to develop my Bash skills, programming in it for me is error-prone and time-consuming.

So recently I have started experimenting with scripting in Common Lisp. It turns out to be very, very easy and convenient if you already know CL. I usually start scripting with a skeleton that looks like this:

(cl:defpackage #:script
  (:use #:cl)
  (:export #:run))

(cl:in-package :script)

;;; code comes here

I run things from SLIME and experiment with the REPL. When I think that I am approaching a solution, I write the main function run:

(defun run ()

Then I use the following Makefile to compile it with cl-launch:

myscript: myscript.lisp
        cl-launch --lisp sbcl --file myscript.lisp --dump '!'   \
        --output myscript -i '(script:run)'

You can of course also include ASDF libraries, check the help of cl-launch.

For interfacing with the environment (pathnames, files, OS, etc), I find the UIOP compatibility library really helpful. It is included with ASDF, so most likely it is already on your computer.