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In my time I’ve written a lot of throwaway binary TCP/IP services (servers and services you can interact with over an internet connection, through command line interface or GUI). For me, this involves designing a protocol from scratch every time with varying levels of hand-rolled authentication and error detection (Send this byte for this command, this byte for this other command, etc.). Once I design the protocol, I then have to write both the command line client and the server — something I usually do from scratch over the raw TCP streams.

This process was fun (and informative) the first few times I did it, but spinning it up from scratch again every time discouraged me from doing it very often. However, thankfully, with the servant haskell library (and servant-cli, for command line clients), writing a TCP server/client pair for a TCP service (using HTTP under the hood) becomes dead-simple — the barrier for creating one fades away that designing/writing a service becomes a tool that I reach for immediately in a lot of cases without second thought.

servant is usually advertised as a tool for writing web servers, web applications, and REST APIs, but it’s easily adapted to write non-web things as well. Let’s dive in and write a simple TCP/IP service (a todo list manager) to see how straightforward the process is!

To goal of this article is to take service/program that you already have planned out, and easily provide it with a networked API that can be used over any TCP/IP connection (over the internet, or even locally). This won’t teach you how to write a todo app, but rather how to hook up a todo app over a TCP/IP connection quickly, with a command line client — and in such a simple way that you wouldn’t give a second thought based on complexity issues.

This post can also serve as a stepping-stone to a “microservices architecture”, if you intend to build towards one (this is explored deeper by k-bx)…but really it’s more focused for standalone user-facing applications. How you apply these techniques is up to you :)

All of the code in this article is available online, and the server and client are available as “stack executables”: if you download them all, and set the permissions properly (chmod u+x), you can directly run them to launch the server and client (if they are all download to the same directory).

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functor-combinators: hackage / github

(Note: This post has been heavily revised to reflect the functor-combinators-0.2 refactoring, as of November 2019. For reference, the original post is available on github.)

(Note 2: The section on contravariant functor combinators was added following the release of functor-combinators-0.3 in August 2020, which added support for contravariant and invariant functor combinators.)

Recently I’ve been very productive what I have been calling the “Functor Combinator” design pattern. It is heavily influenced by ideas like Data types a la Carte and unified free monoidal functors, but the end goal is slightly different in spirit. The goal is to represent schemas, DSL’s, and computations (things like parsers, things to execute, things to consume or produce data) by assembling “self-evident” basic primitives and subjecting them to many different successive transformations and combiners (through combinators, free structures, tensors, and other options). The process of doing so:

1. Forces you to make explicit decisions about the structure of your computation type as an ADT.
2. Allows you to retain isolation of fundamental parts of your domain as separate types
3. Lets you manipulate the structure of your final computation type through normal Haskell techniques like pattern matching. The structure is available throughout the entire process, so you can replace individual components and values within your structure.
4. Allows you to fully reflect the structure of your final computation through pattern matching and folds, so you can inspect the structure and produce useful summaries.

Like “data types a la carte” and free monad/applicative/alternative designs, these techniques allow you to separate the assembly and inspection of your programs from the “running” of them.¹ However, the main difference is that here we focus not just on products and sums, but many different varied and multi-purpose combinators — a “zoo” of combinators. The fixed point is not the end goal. The actual ADT data types themselves are the goal.

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Today, we’re going to implement applicative regular expressions and parsers (in the style of the regex-applicative library) using free structures!

Free structures are some of my favorite tools in Haskell, and I’ve actually written a few posts about them before, including this one using free groups, this one on a free monad variation, and this one on a “free” applicative on a monoid.

Regular expressions (and parsers) are ubiquitous in computer science and programming, and I hope that demonstrating that they are pretty straightforward to implement using free structures will help you see the value in free structures without getting too bogged down in the details!

All of the code in this post is available online as a “stack executable”. When you run it (./regexp.hs), you’ll load up a ghci session with all of the definitions in scope, so you can play around with the functions and types :)

This post should be accessible to late beginner or early intermediate Haskell users, and requires some basic familiarity with pattern matching, algebraic data types, and abstractions like Monoid and Functor, and do notation.

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Not too long ago, I was browsing the prequel memes subreddit — a community built around creative ways of remixing and re-contextualizing quotes from the cinematic corpus of the three Star Wars “prequel” movies — when I noticed that a fad was in progress constructing tries based on quotes as keys indexing stills from the movie corresponding to those quotes.

This inspired me to try playing around with some tries myself, and it gave me an excuse to play around with recursion-schemes (one of my favorite Haskell libraries). If you haven’t heard about it yet, recursion-schemes (and the similar library data-fix) abstracts over common recursive functions written on recursive data types. It exploits the fact that a lot of recursive functions for different recursive data types all really follow the same pattern and gives us powerful tools for writing cleaner and safer code, and also for seeing our data types in a different light. The library is a pathway to many viewpoints — some considered to be particularly natural.

Recursion schemes is a perfect example of those amazing accidents that happen throughout the Haskell ecosystem: an extremely “theoretically beautiful” abstraction that also happens to be extremely useful for writing industrially rigorous code.

Is it possible to learn this power? Yes! As a fun intermediate-level Haskell project, let’s build a trie data type in Haskell based on recursion-schemes to see what it has to offer!

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(TL;DR: scroll down to the very bottom for a summary and the closed form solution)

Another short Advent of Code post! Advent of Code 2018 is in full swing; we’re 40% of the way through. Every once in a while, if I find a fun way to solve a problem, I’ll make a short post about it. You can check out my other ones here on the series page, and you can also find my daily reflections here, as well. And, again, if you’re following along in Haskell, why not hop on glguy’s semi-official Haskell Leaderboard (join code 43100-84040706)! There are also Haskellers on freenode ##adventofcode, and also #adventofcode on the Functional Programming slack. You might also find my advent of code api haskell bindings helpful too!

Today, we’re going to be using linear algebra, calculus, and galilian transformations to solve the Day 10 challenge. (That’s right, this isn’t just a Haskell blog, I do have math posts on occasion too :) )

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Hi all! If you don’t already know, Advent of Code is in full swing this year! If you’re participating and using Haskell, you’re welcome to join us at glguy’s semi-official Haskell Leaderboard (join code 43100-84040706)! There are also Haskellers on freenode ##adventofcode, and also #adventofcode on the Functional Programming slack. I also wrote a haskell library to the API, if you’re looking to streamline your process!

My daily reflections are online, where I talk about how I approach each problem and what insight purely typed Functional Programming gives us for each problem.

Every once in a while I’ll find a challenge that I think can be pulled out as a short blog post, so I’ll bring them onto the blog for a more long-term sort of longevity!¹

In this one, I’ll talk about using group theory to solve the Day 5 challenge. Spoilers for those who have not solved it yet!

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Hi again! Welcome back; let’s jump right into the fourth and final part of our journey through the singleton design pattern and the great singletons library.

Please check out the first three parts of the series and make sure you are comfortable with them before reading on. I definitely also recommend trying out some or all of the exercises, since we are going to be building on the concepts in those posts in a pretty heavy way.

Today we’re going to jump straight into functional programming at the type level. Code in this post is built on GHC 8.6.1 with the nightly-2018-09-29 snapshot (so, singletons-2.5). However, unless noted, all of the code should still work with GHC 8.4 and singletons-2.4.

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Welcome back! This article is part 3 of our journey through the singleton design pattern, and the great singletons library!

This post will be a continuation of Part 1 and Part 2, so if you haven’t read those first, now would be a good time to pause and do so (and also try to complete the exercises). Today we will be expanding on the ideas in those posts by working with more complex ways to restrict functions based on types. Like the previous posts, we will start by writing things “by hand”, and then jumping into the singletons library and seeing how the framework gives you tools to work with these ideas in a smoother way.

The first half of today’s post will introduce a new application and design pattern that the usage of singletons greatly enhances. The second part of today’s post deals directly with the lifting of functions to the type level, which is made practical by the usage of singletons and the singletons library.

Code in this post is built on GHC 8.6.1 with the nightly-2018-09-29 snapshot (so, singletons-2.5). However, unless noted, all of the code should still work with GHC 8.4 and singletons-2.4. Again, you can download the source for this file here, and, if stack is installed, you can drop into a ghci session with all of the bindings in scope executing it:

$ ./Door3.hs

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