Justin Meiners justinmeiners

Reader Macros in Common Lisp

This post also appears on lisper.in.

Reader macros are perhaps not as famous as ordinary macros. While macros are a great way to create your own DSL, reader macros provide even greater flexibility by allowing you to create entirely new syntax on top of Lisp.

Paul Graham explains them very well in [On Lisp][] (Chapter 17, Read-Macros):

The three big moments in a Lisp expression's life are read-time, compile-time, and runtime. Functions are in control at runtime. Macros give us a chance to perform transformations on programs at compile-time. ...read-macros... do their work at read-time.

Description

This simple script will take a picture of a whiteboard and use parts of the ImageMagick library with sane defaults to clean it up tremendously.

The script is here:

#!/bin/bash
convert "$1" -morphology Convolve DoG:15,100,0 -negate -normalize -blur 0x1 -channel RBG -level 60%,91%,0.1 "$2"

Results

Hofstadter on Lisp

In the mid-80s, while reading through my roommate's collection of Scientific American back issues, I encountered this introduction to Lisp written by Douglas Hofstadter. I found it very charming at the time, and provide it here (somewhat illegally) for the edification of a new generation of Lispers.

In a testament to the timelessness of Lisp, you can still run all the examples below in emacs if you install these aliases:

(defalias 'plus #'+)
(defalias 'quotient #'/)
(defalias 'times #'*)
(defalias 'difference #'-)

	/*
	* This file is part of the XXX distribution (https://github.com/xxxx or http://xxx.github.io).
	* Copyright (c) 2015 Liviu Ionescu.
	*
	* This program is free software: you can redistribute it and/or modify
	* it under the terms of the GNU General Public License as published by
	* the Free Software Foundation, version 3.
	*
	* This program is distributed in the hope that it will be useful, but
	* WITHOUT ANY WARRANTY; without even the implied warranty of

	// NOTE: This is now rolled up in a package and supports more scenarios: https://github.com/dsherret/using-statement

	interface IDisposable {
	dispose();
	}

	function using<T extends IDisposable>(resource: T, func: (resource: T) => void) {
	try {
	func(resource);
	} finally {

	(require 'dbg)

	(global-set-key (kbd "<C-S-f5>") 'dbg-restart)
	(global-set-key (kbd "<f5>") 'dbg-continue)
	(global-set-key (kbd "<f9>") 'dbg-toggle-breakpoint)
	(global-set-key (kbd "<f8>") 'dbg-watch)
	(global-set-key (kbd "<f10>") 'dbg-next)
	(global-set-key (kbd "<C-f10>") 'dbg-continue-to-here)
	(global-set-key (kbd "<f11>") 'dbg-step)
	(global-set-key (kbd "<C-S-f10>") 'dbg-jump)

	; The official HD AI
	; An Artificial Intelligence Script written by Archon and Promiskuitiv
	; Get in contact with Promiskuitiv by sending a mail to [email protected]


	; List of taunts it reacts to:

	; Standard taunts.
	; 33 - Stop slinging resources. If slinging is requested early and is immediately canceled it may mess up the strategy.
	; 38 - Sling Resources. Human player only, stops any unit production except for civilian units.

	/*
	2D Angle Interpolation (shortest distance)

	Parameters:
	a0 = start angle
	a1 = end angle
	t = interpolation factor (0.0=start, 1.0=end)

	Benefits:
	1. Angles do NOT need to be normalized.

	# Originally from http://sharebear.co.uk/blog/2009/09/17/very-simple-python-caching-proxy/
	#
	# Usage:
	# A call to http://localhost:80000/example.com/foo.html will cache the file
	# at http://example.com/foo.html on disc and not redownload it again.
	# To clear the cache simply do a `rm *.cached`. To stop the server simply
	# send SIGINT (Ctrl-C). It does not handle any headers or post data.

	import BaseHTTPServer
	import hashlib