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Racket provides a simple yet flexible web framework for serving dynamic web content. This post covers the basics for using the low-level layer of that framework, including: using low-level responses using X-Expression responses enabling SSL enabling basic authentication handling form content serving static files I’ve combined all of these concepts together to create a “minimum viable” academic wik

You type in commands. Bash executes them. Unix users spend a lot of time manipulating files at the shell. As a shell, it is directly available via the terminal in both Mac OS X (Applications > Utilities) and Linux/Unix. At the same time, bash is also a scripting language: Bash scripts can automate routine or otherwise arduous tasks involved in systems administration. Why use bash? Here are example

There is a pattern with students learning functional programming. First, they try to use loops and mutation; this ends with awkward, broken programs. There is confusion and aggravation. Even hostility. Eventually, they accept and embrace recursion. But, then they write too much. While recursion is better than iteration for functional programming, new functional programmers are unaware of powerful

A colleague recently asked me for advice on work-life balance. My reply crossed the threshold for a blog post, so I'm posting it here to be consistent with my own principles on academic blogging. My high-level tips are to: Explicitly acknowledge priorities Avoid workaholism Avoid perfectionism Set and enforce boundaries Avoid over-commitment Use a work-flow system Communicate Keep hobbies Exploit

Balanced-tree-based maps are a workhorse in functional programming. Because of their disarming simplicity, Chris Okasaki's purely functional red-black trees are a popular means for implementing such maps. In his book, self-balancing binary search trees are less than a page of code. Except that delete is left as an exercise to the reader. Unfortunately, deletion is tricky. Stefan Kahrs devised a wi

Church encodings, the U combinator and the Y combinator in JavaScript The lambda-calculus is almost too small to be a Turing-complete language. Almost. Because it is Turing-complete. With just three expression forms -- variable references, function application and anonymous functions -- it is possible to express arbitrary computation. These core forms exist as a subset of many popular programming

JavaScript is a Gestalt language. One's sentiment toward JavaScript flips between elegance and disgust without transiting intermediate states. The key to seeing JavaScript as elegant is understanding its warts, and knowing how to avoid, work around or even exploit them. I adopted this avoid/fix/exploit approach after reading Doug Crockford's JavaScript: The Good Parts: Doug has a slightly differen

SSH is a protocol for authenticating and encrypting remote shell sessions. But, using SSH for just remote shell sessions ignores 90% of what it can do. This article covers less common SSH use cases, such as: using passwordless, key-based login; setting up local per-host configurations; exporting a local service through a firewall; accessing a remote service through a firewall; setting up a SOCKS p

Sculpting text with regex, grep, sed, awk, emacs and vim Unix is an alliance of loosely structured text files bound together and governed by scripts. Unix is the United Confederation of Strings: The string is a stark data structure and everywhere it is passed there is much duplication of process. It is a perfect vehicle for hiding information. --Alan Perlis Tools built in the Unix tradition excel

Relational algebra is a theory for manipulating relations whose power is equivalent to SQL and relational calculus. Remarkably, relational algebra has only six primitive operations. I define the six primitives below, but if you're looking for a comprehensive work on relational theory, particularly as it relates to modern databases, I recommend Date's SQL and Relational Theory: Relations in Unix Ma

Unix plays a supporting role in all of the classes I teach, but it is essential in my Scripting Language Design and Implementation course. Unix is an operating ecosystem for executing and composing scripts. It ranks high on my list of what every CS major should know. Unfortunately, students no longer arrive familiar with Unix-style computing, and most are not willing to learn it on their own. Some

It’s important for programmers to continuously challenge themselves in order to avoid creative and technical stagnation. In the spirit of the new year, back in 2012, I compiled twelve month-sized resolutions to help avoid that stagnation. I’ve updated that list for 2023. Each month is an annually renewable technical or personal challenge: Go analog. Stay healthy. Embrace the uncomfortable. Automat

Translating math into code with examples in Java, Racket, Haskell and Python Discrete mathematical structures form the foundation of computer science. These structures are so universal that most research papers in the theory of computation, programming languages and formal methods present concepts in terms of discrete mathematics rather than code. The underlying assumption is that the reader will

Given the expansive growth in the field, it's become challenging to discern what belongs in a modern computer science degree. My own faculty is engaging in this debate, so I've coalesced my thoughts as an answer to the question, "What should every computer science major know?" I've tried to answer this question as the conjunction of four concerns: What should every student know to get a good job?

Academics rarely release code, but I hope a license can encourage them. Generally, academic software is stapled together on a tight deadline; an expert user has to coerce it into running; and it's not pretty code. Academic code is about "proof of concept." These rough edges make academics reluctant to release their software. But, that doesn't mean they shouldn't. Most open source licenses (1) requ

The problem with email is that people think it's electronic mail. Email is not mail in electronic form. You are not writing a letter. Few send readable email or tap the deliberative potential of the medium. For example, email should be formatted into points--not paragraphs. And, you should not always reply above the message you were sent. Many of the tips below are already widely followed in acade

What is lambda? Syntactically, lambda refers to a form for describing anonymous functions. But, a lambda does not become a function pointer. It becomes a closure. Closures are data structures with both a code and a data component. There are two dominant strategies for compiling lambdas into closures: flat closures and linked (or shared) closures. It is possible to understand both strategies in ter

In terms of unreasonable effectiveness, the transformation to continuation-passing style (CPS) ranks with the Y combinator. During compilation, high-level control constructs ranging from coroutines and exceptions to while loops and break statements steadily desugar into a mixture of two constructs: calls to functions and calls to continuations. Then, during CPS transformation, continuations desuga

HOWTO: Create native-looking iPhone/iPad applications from HTML, CSS and JavaScript Though it's not widely known, you can write native-feeling iOS apps for the iPhone and iPad in JavaScript (+ HTML and CSS). In this article, I'll explain how to: strip away the browser chrome (the url bar and button bar); prevent viewport scrolling and scaling; respond to multi-touch and gesture events; use webkit

In examples, we'll still use juxtaposition to denote concatenation. The derivative for regular languages The useful property of the derivative from an implementation point of view is that it has a recursive definition over regular languages: Dc(∅) = ∅ Dc(ε) = ∅ Dc(c) = ε Dc(c') = ∅ if c is not c' Dc(A ○ B) = Dc(A) ○ B if A does not contain the empty string Dc(A ○ B) = Dc(A) ○ B ∪ Dc(B) if A contai

Fixed-point combinators in JavaScript: Memoizing recursive functions It comes as a surprise to many programmers that it is possible to express a "recursive" function like factorial without using recursion or iteration. The technique involved is subtle but powerful: the recursive function is computed as the "fixed point" of a non-recursive function. To compute the fixed point, we can use the Y comb

Continuation-passing style (CPS) originated as a style of programming in the 1970s, and it rose to prominence as an intermediate representation for compilers of advanced programming languages in the 1980s and 1990s. It's now being rediscovered as a style of programming for non-blocking (usually distributed) systems. There's a warm spot in my heart for CPS, because it was the secret weapon in my Ph

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