# in Code

## Recent Entries (Page 3)

• ### The Const Applicative and Monoids

The Applicative typeclass has a somewhat infamous reputation for having opaque laws. There are a lot of great alternative rephrasing of these laws, from many different approaches. For this post, however, I want to talk about Applicative in terms of one of my favorites: Const.

• ### You Could Have Invented Matrices!

You could have invented matrices!

Let’s talk about vectors. A vector (denoted as $\mathbf{x}$, a lower-case bold italicized letter) is an element in a vector space, which means that it can be “scaled”, like $c \mathbf{x}$ (the $c$ is called a “scalar” — creative name, right?) and added, like $\mathbf{x} + \mathbf{y}$.

In order for vector spaces and their operations to be valid, they just have to obey some common-sense rules (like associativity, commutativity, distributivity, etc.) that allow us to make meaningful conclusions.

• ### Introducing the backprop library

backprop: hackage / github

I’m excited to announce the first official release of the backprop library (currently at version 0.1.3.0 on hackage)! backprop is a library that allows you write functions on your heterogeneous values like you would normally and takes them and (with reverse-mode automatic differentiation) automatically generate functions computing their gradients. backprop differs from the related ad by working with functions using and transforming different types, instead of only one monomorphic scalar type.

This has been something I’ve been working on for a while (trying to find a good API for heterogeneous automatic differentiation), and I’m happy to finally find something that I feel good about, with the help of a lens-based API.

As a quick demonstration, this post will walk through the creation of a simple neural network implementation (inspired by the Tensorflow Tutorial for beginners) to learn handwritten digit recognition for the MNIST data set. To help tell the story, we’re going to be implementing it “normally”, using the hmatrix library API, and then re-write the same thing using backprop and hmatrix-backprop (a drop-in replacement for hmatrix).

• ### "Interpreters a la Carte" in Advent of Code 2017 Duet

This post is just a fun one exploring a wide range of techniques that I applied to solve the Day 18 puzzles of this past year’s great Advent of Code. The puzzles involved interpreting an assembly language on an abstract machine. The twist is that Part A gave you a description of one abstract machine, and Part B gave you a different abstract machine to interpret the same language in.

This twist (one language, but different interpreters/abstract machines) is basically one of the textbook applications of the interpreter pattern in Haskell and functional programming, so it was fun to implement my solution in that pattern — the assembly language source was “compiled” to an abstract monad once, and the difference between Part A and Part B was just a different choice of interpreter.

Even more interesting is that the two machines are only “half different” – there’s one aspect of the virtual machines that are the same between the two parts, and aspect that is different. This means that we can apply the “data types a la carte” technique in order to mix and match isolated components of virtual machine interpreters, and re-use code whenever possible in assembling our interpreters for our different machines! This can be considered an extension of the traditional interpreter pattern: the modular interpreter pattern.

This blog post will not necessarily be a focused tutorial on this trick/pattern, but rather an explanation on my solution centered around this pattern, where I will also add in insight on how I approach and solve non-trivial Haskell problems. We’ll be using the operational package to implement our interpreter pattern program and the type-combinators package to implement the modularity aspect, and along the way we’ll also use mtl typeclasses and classy lenses.

The source code is available online and is executable as a stack script. This post is written to be accessible for early-intermediate Haskell programmers.

• ### Introduction to Singletons (Part 2)

Welcome back to our journey through the singleton design pattern and the great singletons library!

This post is a direct continuation of Part 1, so be sure to check that out first if you haven’t already! If you hare just jumping in now, I suggest taking some time to to through the exercises if you haven’t already!

Again, code 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. All of the code is also available here, and you can drop into a ghci session with all of the bindings in scope by executing the file:

\$ ./Door2.hs
• ### Introduction to Singletons (Part 1)

Real dependent types are coming to Haskell soon! Until then, we have the great singletons library :)

If you’ve ever run into dependently typed programming in Haskell, you’ve probably encountered mentions of singletons (and the singletons library). This series of articles will be my attempt at giving you the story of the library, the problems it solves, the power that it gives to you, and how you can integrate it into your code today!1 (Also, after my previous April Fools post, people have been asking me for an actual non-joke singletons post)

This post (Part 1) will go over first using the singleton pattern for reflection, then introducing how the singletons library helps us. Part 2 will discuss using the library for reification, to get types that depend on values at runtime. Part 3 will go into the basics of promoting functions values to become functions on types in a usable, and Part 4 will go deeper into the lifting of functions, using singleton’s defunctionalization scheme to utilize the higher-order functions we love at the type level. Part 3 will go into the basics singleton’s

I definitely am writing this post with the hope that it will be obsolete in a year or two. When dependent types come to Haskell, singletons will be nothing more than a painful historical note. But for now, singletons might be the best way to get your foot into the door and experience the thrill and benefits of dependently typed programming today!

1. This series will be based on a talk I gave over the summer, and will expand on it.↩︎

• ### Advent of Code 2017! Ongoing solutions and explanations

Just a short post to share that I started a github repository of my Advent of Code 2017 Solutions, as I write them!

I also am including my reflections and explanations on my solutions, explaining my thought processes and how the solutions work.

Yes I definitely spent a bit too much time writing the executable, which is an automated (cached) downloader, test suite runner (on sample inputs), and benchmark suite.

I originally was only going to casually try the problems (like I did last year), but I hit a decent global rank by accident on Day 4 (which was very suited for Haskell!), and since then I’ve been taking things seriously to try to aim for the global leaderboard (top 100). This is a struggle for me because I’m not really the fastest algorithm person, but I think it’s a fun goal for me to try to hit this year.

Wish me luck! And if you haven’t started yet, it’s not too late to join in the fun! glguy has been maintaining the semi-official Haskell Leaderboard (join code 43100-84040706) – come join us!

• ### Hamiltonian Dynamics in Haskell

As promised in my hamilton introduction post (published almost exactly one year ago!), I’m going to go over implementing of the hamilton library using

1. DataKinds (with TypeLits) to enforce sizes of vectors and matrices and help guide us write our code
2. Statically-sized linear algebra with hmatrix