James Earl Douglas

http4s

Sun, 31 Aug 2025 00:00:00 GMT

http4s is a Scala library for building purely functional HTTP servers and clients in the Tagless Final style using Cats Effect.

APRSdroid

Sun, 20 Apr 2025 00:00:00 GMT

Let's make an inexpensive APRS rig for portable position tracking and messaging using a cheap handheld transciever and an old Android phone.

___ \|/ | | NBFM | | | .-------. | | | .----. | | | |-------------------------->| Phone | | HT | AFSK | | | |<--------------------------| | '----' '-------'

Functional Idempotence

Thu, 22 Aug 2024 00:00:00 GMT

Idempotence is often defined as:

∀x : f(f(x)) = f(x)

But this way makes more sense to me:

∀s,x : f(s, x) = (s, y)

Here's how I like to think about it.

Slop-Driven Development

Fri, 16 Aug 2024 00:00:00 GMT

 .-----------. .-------. | | | Model |------>| | '-------' | | .---------. | GenAI |----->| Garbage | .--------. | | '---------' | Prompt |----->| | '--------' | | '-----------'

AI-assisted code generation is just an expensive way to create technical debt.

F[unctional] Core, IO[mperative] Shell

Sat, 04 May 2024 00:00:00 GMT

Let's design a service with an architecture that we'll call Semantic Layering.

Our service will be a simple a hit counter. Our architecture will draw inspiration from Functional Domain Modelling and The Clean Architecture, and will encode effects using Tagless-Final.

We'll separate pure code into lower layers, used by impure code at higher layers:

- runners --------------------------------o-- | - drivers -----------------------------o--x-- | | impure | | =======================================|==|== pure | | | | - use-cases ------------------------o--x--x-- | | | - domain ---------------------------v--v--v--

Ascension from Dependency Hell

Sun, 10 Dec 2023 00:00:00 GMT

Scala has a rapidly-changing ecosystem of interconnected libraries. This tends to lead to dependency hell, in which a project's transitive dependency graph contains version mismatches:

 .--------. | Logger | | * v1 <----. .--> * v2 | | | '--------' | | | .------------. .-------. | Multiplier | | Adder | '------------' '-------' ^ ^ | .------. | '---| Main |---' '------'

Not only is this painful to manage, but current approaches leave projects vulnerable to defects that can be hard to catch in time.

We explore shading as a simple yet robust approach to deconflicting transitive dependencies in Scala projects.

Data Architectures

Tue, 28 Dec 2021 00:00:00 GMT

This is a collection of notes on a variety of data architectures.

Linear Types in Scala

Fri, 21 May 2021 00:00:00 GMT

linear-scala is an sbt plugin that adds support for linear types in Scala via a custom Scalafix linter.

Spark

Wed, 11 Mar 2020 00:00:00 GMT

Spark is a MapReduce framework for Scala.

Dire Wolf

Fri, 19 Apr 2019 00:00:00 GMT

Dire Wolf is a software terminal node controller (TNC). Let's use it to send and receive APRS trafic using a handheld transceiver and an RTL-SDR receiver.

___ ___ \|/ \|/ | | | NBFM .-----------. | NBFM | | | | | | Dire Wolf | .----. | | | | | .---------. IQ '-----------' AFSK | HT | | RTL-SDR |------------->\ - - - - - \--------------->| | '---------' \ - - - - - \ '----' '-----------'

Microservices

Wed, 10 Apr 2019 00:00:00 GMT

Microservices architectures foster rapid development and deployment of incremental improvements. This makes it easy (and fun) to build an expanding suite of software in a patchwork of available time.

Functional Electromagnetism

Sat, 30 Mar 2019 00:00:00 GMT

Strengthen your understanding of functional programming by looking at it from a fresh and unconventional perspective.

___ \|/ | .-----------. | | | | | GNU Radio | | | | | .--------. '-----------' '-------->| HackRF |----->\ - - - - - \ '--------' \ - - - - - \ '-----------'

We use GNU Radio to examine digital signal processing systems, and explore how we can use our understanding of functional programming to reason about unfamiliar systems such as software-defined radio by looking through the lens of category theory.

ZIO

Sun, 25 Nov 2018 00:00:00 GMT

ZIO is a library (or perhaps a framework) for building effect-oriented programs in Scala.

Doobie

Wed, 25 Oct 2017 00:00:00 GMT

Doobie is a Scala library for working with Java's JDBC API in a purely functional style.

Argonaut

Thu, 17 Aug 2017 00:00:00 GMT

Argonaut is Scala library for parsing, serializing, and deserializing JSON in a purely functional style.

Tanks a Lot

Sun, 21 May 2017 00:00:00 GMT

A minimally-multiplayer experiment using WebSockets and JavaScript.

Haskeval

Thu, 11 Aug 2016 00:00:00 GMT

Let's build a little Haskell interpreter that we can use as a safe runhaskell as a service on the Web.

Codedown

Sun, 29 May 2016 00:00:00 GMT

Codedown is a Node.js library and command line utility to extract code blocks from Markdown files. It can be used to help check the correctness of code in articles and slideshows, or even to allow Markdown to be the distribution format for a project's source code.

Scotty

Fri, 04 Mar 2016 00:00:00 GMT

Scotty is a Haskell framework for building Web apps in the Sinatra style.

Scalaz

Sat, 03 Oct 2015 00:00:00 GMT

Scalaz is a library for functional programming in Scala.

AuthZ

Fri, 11 Sep 2015 00:00:00 GMT

This is a collection of notes on access control design patterns.

Sad Trombone

Wed, 29 Jul 2015 00:00:00 GMT

Womp womp.

Leon

Sat, 25 Jul 2015 00:00:00 GMT

Leon is a software verification system for Scala. It uses a limited subset of Scala, called Pure Scala, and a nifty online evaluation envrionment to statically verify programs.

Python

Tue, 21 Jul 2015 00:00:00 GMT

This is a collection of notes on the Python programming language and ecosystem.

MediaWiki

Fri, 10 Jul 2015 00:00:00 GMT

This is a collection of notes on MediaWiki development and administration.

TypeScript

Tue, 23 Jun 2015 00:00:00 GMT

This is a collection of notes on the TypeScript programming language.

Specified Versioning

Wed, 17 Jun 2015 00:00:00 GMT

This is a guide to choosing version numbers for software projects that designate tests as specification.

Nix is awesome, and you should too

Fri, 05 Jun 2015 00:00:00 GMT

Nix is a functional language, package manager, operating system, DevOps tool, and ecosystem.

Wikidata Explorer

Mon, 18 May 2015 00:00:00 GMT

Traverse Wikidata as an interactive visual knowledge graph.

Elasticsearch

Thu, 12 Mar 2015 00:00:00 GMT

Elasticsearch is a full-text search engine based on Apache Lucene.

RDF

Fri, 20 Feb 2015 00:00:00 GMT

This is a collection of notes on the Semantic Web.

Java

Thu, 12 Feb 2015 00:00:00 GMT

This is a collection of notes on the Java programming language.

Spectre Detector v1.0

Thu, 12 Feb 2015 00:00:00 GMT

Cheaper than a GhostArk, but just as accurate. Inspired by Steven Novella in SGU episode #500.

Marky Mark

Wed, 11 Feb 2015 00:00:00 GMT

An underpowered in-browser Markdown editor using Marked.

Ray Casting

Sun, 14 Dec 2014 00:00:00 GMT

I have no idea what I'm doing.

JavaScript

Thu, 25 Sep 2014 00:00:00 GMT

This is a collection of notes on the JavaScript programming language, libraries, tools, and ecosystem.

Gravity

Fri, 19 Sep 2014 00:00:00 GMT

GmM/r^2

AspectJ

Sat, 21 Jun 2014 00:00:00 GMT

AspectJ is a Java extension for adding aspected-oriented programming (AOP) support.

Maven

Mon, 09 Jun 2014 00:00:00 GMT

Maven is a build tool for Java.

Standard ML

Thu, 17 Oct 2013 00:00:00 GMT

This is a collection of notes on the Standard ML programming language.

Integrated Development Environment

Sat, 14 Sep 2013 00:00:00 GMT

All you need to build software is a terminal emulator (e.g. xterm), a text editor (i.e. vim), and your language's build tool (e.g. sbt, mvn, make, cabal, npm).

In this talk, we show a simple yet comprehensive way to write Scala using just vim and sbt.

Programming Exercises

Wed, 31 Jul 2013 00:00:00 GMT

These programming exercises have helped me keep sharp over the years.

The Mechanism Manifesto

Tue, 11 Jun 2013 00:00:00 GMT

This is a collection of notes on a variety of techniques for making it easier to do the right thing and harder to do the wrong thing by mechanizing intentions.

Effect Systems

Fri, 16 Nov 2012 00:00:00 GMT

An effect system is a mechanism for separating the semantics of a program from its specification by using effects.

An effect is an operation, behavior, pattern, signature, or computational context of which meaning is defined by an interpreter.

Effect systems are made up of:

Effect types, which can be encoded as a generic with a single unbound parameter such as IO[_], Option[_], Future[_], etc.
Effect operations that use and produce effect types
Effect handlers that give meaning to effect operations

In this post, we explore different ways to implement effect systems in Scala, beginning by describing an effectful program that we would like to write as a pure value, and then writing different toy effect systems that can run it.

C

Fri, 22 Jun 2012 00:00:00 GMT

This is a collection of notes on the C programming language.

Functional Refactoring

Sat, 16 Jun 2012 00:00:00 GMT

Refactoring provides an accessible opportunity to learn about imperative and functional design patterns, and Scala's hybrid OO/FP design caters to both.

We explore examples of Scala code written using familiar imperative design patterns, and refactor each one using a counterpart functional pattern. We learn how to replace mutable variables with the state monad, loops with folds, thrown exceptions with sum types, dependency injection with the reader monad, and much more. As we go, we build a mapping of corresponding imperative and functional patterns.

Type Classes

Sun, 11 Mar 2012 00:00:00 GMT

Scala supports ad-hoc polymorphism via traits, context bounds, and implicits.

We explore an approach to implementing type classes in Scala.

Haskell

Sat, 18 Feb 2012 00:00:00 GMT

This is a collection of notes on the Haskell programming language.

Delimited Continuations

Mon, 20 Jun 2011 00:00:00 GMT

This is an introduction to delimited continuations in Scala, and a sample of some of its practical applications.

reset { println("What is your name?") val name = shift { k: (String => Unit) => val name = readLine() k(name) } println(s"Hello, ${name}!") }

Akka

Sat, 19 Mar 2011 00:00:00 GMT

Akka is an actor library for Scala.

Scala

Sun, 27 Feb 2011 00:00:00 GMT

This is a collection of notes on the Scala programming language.

Spring MVC

Sat, 02 Oct 2010 00:00:00 GMT

This post is about Java Web application development using the Spring MVC framework. It distills much of what I have learned from developing enterprise applications with Spring MVC, guiding usage of the components of Spring MVC that I most frequently encounter in practice and on discussion forums. This post provides a brief overview of these components by taking you through the development of an example Spring MVC application from scratch.

My goals in writing this post are to guide developers who are unfamiliar with Spring MVC, and to supply a convenient reference to more seasoned developers.

JCode

Fri, 04 Jun 2010 00:00:00 GMT

JCode is a simple rules-based substitution cypher for classroom mischief.

Spring Web Services

Sun, 13 Sep 2009 00:00:00 GMT

Spring Web Services (Spring-WS) is a framework for writing SOAP Web services.

Krypto

Fri, 11 Sep 2009 00:00:00 GMT

Two minus three equals negative fun!

Spring AOP

Mon, 31 Aug 2009 00:00:00 GMT

Spring AOP is a framework for wiring crosscutting concerns into applicable functionality without intrusion into the source code.

Guice

Sat, 22 Aug 2009 00:00:00 GMT

Guice is a dependency injection framework for Java.

Jetty

Sat, 18 Jul 2009 00:00:00 GMT

Jetty is servlet container that is simple and easy to work with for testing or developing embedded web applications. It simple yet capable compared to alternatives such as JBoss or Tomcat. My favorite "feature" is that Jetty consists entirely of two JAR files, coming in at just under 700 KB.

Hibernate

Wed, 18 Mar 2009 00:00:00 GMT

Hibernate is an ORM for Java.

Spring Framework

Mon, 16 Mar 2009 00:00:00 GMT

The Spring Framework is a dependency injection framework for Java.