serialhex

A composable pattern for pure state machines with effects

State machines are everywhere in interactive systems, but they're rarely defined clearly and explicitly. Given some big blob of code including implicit state machines, which transitions are possible and under what conditions? What effects take place on what transitions?

There are existing design patterns for state machines, but all the patterns I've seen complect side effects with the structure of the state machine itself. Instances of these patterns are difficult to test without mocking, and they end up with more dependencies. Worse, the classic patterns compose poorly: hierarchical state machines are typically not straightforward extensions. The functional programming world has solutions, but they don't transpose neatly enough to be broadly usable in mainstream languages.

Here I present a composable pattern for pure state machiness with effects,

Interpreters/Compilers

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.

taken from

The system

The minisystem goes like this:

Say what you do and roll a number of d6s.
If the sum of your roll is higher than the opposing roll (either another player or the DM), the thing you wanted to happen, happens.
The number of the d6s you roll is determined by the level of skill you have.

Mathematics: What do grad students in math do all day?

by Yasha Berchenko-Kogan

A lot of math grad school is reading books and papers and trying to understand what's going on. The difficulty is that reading math is not like reading a mystery thriller, and it's not even like reading a history book or a New York Times article.

The main issue is that, by the time you get to the frontiers of math, the words to describe the concepts don't really exist yet. Communicating these ideas is a bit like trying to explain a vacuum cleaner to someone who has never seen one, except you're only allowed to use words that are four letters long or shorter.

What can you say?

	# Hello, and welcome to makefile basics.
	#
	# You will learn why `make` is so great, and why, despite its "weird" syntax,
	# it is actually a highly expressive, efficient, and powerful way to build
	# programs.
	#
	# Once you're done here, go to
	# http://www.gnu.org/software/make/manual/make.html
	# to learn SOOOO much more.

	#!/bin/bash

	# Usage: sudo apt-get install -yqq curl; curl -L -s https://raw.github.com/gist/4671312/ubuntu-install-media.sh \| sudo bash

	#alias sagi="apt-get install --yes --quiet" # normall sudo apt-get, but this is already root
	#alias sagid="apt-get install --yes --quiet --download-only" # same as above
	#STDOUT=/dev/stdout
	#STDERR=/dev/stderr
	STDOUT=/dev/null
	STDERR=/dev/null

	;; based on core.logic 0.8-alpha2 or core.logic master branch

	(ns sudoku
	(:refer-clojure :exclude [==])
	(:use clojure.core.logic))

	(defn get-square [rows x y]
	(for [x (range x (+ x 3))
	y (range y (+ y 3))]
	(get-in rows [x y])))

	Latency Comparison Numbers (~2012)
	----------------------------------
	L1 cache reference 0.5 ns
	Branch mispredict 5 ns
	L2 cache reference 7 ns 14x L1 cache
	Mutex lock/unlock 25 ns
	Main memory reference 100 ns 20x L2 cache, 200x L1 cache
	Compress 1K bytes with Zippy 3,000 ns 3 us
	Send 1K bytes over 1 Gbps network 10,000 ns 10 us
	Read 4K randomly from SSD* 150,000 ns 150 us ~1GB/sec SSD

	{-# LANGUAGE GADTs #-}
	{-# LANGUAGE RankNTypes #-}
	{-# LANGUAGE StandaloneDeriving #-}
	module RedBlackTree where

	data Zero
	data Succ n
	type One = Succ Zero

	data Black