in Code


Justin Le

Welcome! My name is Justin Le. I’m a PhD candidate at Chapman University in California, studying Computational Science & Applied Maths; I studied Physics and Computer Science at University of California, San Diego :)

This is just my weblog covering my various adventures in programming and explorations in the vast worlds of computation, physics, engineering, mathematics, and knowledge. Expect a healthy curiosity and an only slightly unhealthy obsession for finding new ways to marvel, wonder, and create. Join me if you wish!

Check out some of my most popular articles: First-Class Statements, “IO Monad” Considered Harmful, and Purely Functional Typed Models!

Recent Entries

  • Shifting the Stars: Advent of Code with Galilean Optimization

    (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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  • Alchemical Groups: Advent of Code with Free Groups and Group Homomorphisms

    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!


    1. These short posts won’t be counted as “paid” Patreon posts.

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  • Introduction to Singletons (Part 4)

    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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  • Introduction to Singletons (Part 3)

    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:

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  • Lenses embody Products, Prisms embody Sums

    I’ve written about a variety of topics on this blog, but one thing I haven’t touched in too much detail is the topic of lenses and optics. A big part of this is because there are already so many great resources on lenses.

    This post won’t be a “lens tutorial”, but rather a dive into an perspective on lenses and prisms that I’ve heard repeated many times (usually credited to Edward Kmett, but shachaf has helped me trace the origins back to this paste by Reid Barton) but never quite expanded on in-depth. In particular, I’m going to talk about the perspective of lenses and prisms as embodying the essences of products and sums (respectively), the insights that perspective brings, how well it flows into the “profunctor optics” formulation, and how you can apply these observations to practical usage of lenses and prisms.

    The “final code” in this post is available online as a “stack executable” that, when run, will pop you into a ghci session with all of the final definitions in scope, so you can play around with them :)

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  • Starting a Patreon

    Short version: I’ve started a Patreon page for those who want to donate or help me sustain my writing in my post-doctoral life! :D

    Long version: Hi all! Hope you can excuse the two non-technical and personal posts in a row :) To give some context on this announcement, here’s some news — I graduated*!

    Graduating* from a Doctoral Program at Chapman University
    Graduating* from a Doctoral Program at Chapman University

    *Well, not exactly :) I haven’t finished my dissertation and defended my thesis yet (but expect to in the upcoming months); Chapman University was generous enough to allow me to walk in this past Spring’s ceremony.

    I have a lot of excitement about my future after my defense and official commencement. However, with this excitement comes a lot of uncertainty.

    As a student I have been able to devote time on-and-off again to this blog, which has become a passion of mine. I’ve even managed to find time to live-stream some coding projects occasionally! When I started this blog almost five years ago, I only wanted to write down what I was learning to help myself remember, and to help others going through the same thing as me. I never expected the warm reception from the Haskell and Programming community. Every once in a while I meet people who tell me personally that an article of mine had inspired them to get into functional programming, or had been the final straw to helping them get over a conceptual hurdle. It’s moments like these that really make everything worth it. And, as a researcher in a field dominated by imperative and dynamically typed programming, watching and encouraging the growth of functional programming has brought me a lot of joy.

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  • In Memory of Ertugrul Söylemez (1985 - 2018)

    I was heartbroken to find out recently that Ertugrul Söylemez has passed away suddenly. Many have come forward to express their sadness about this passing and how much this has impacted the Haskell community, and how much of a loss it is for functional programming at large.

    They aren’t wrong; Ertugrul was one of the faces of the friendly, warm, encouraging, patient Haskell teaching that Haskell has grown to be known for. Ertugrul was also one of the original pioneers in the implementation and theory of Functional Reactive Programming and continued to innovate even through this year. His name is now and forever will be synonymous with the “pull-based” variant of functional reactive programming. And the freenode #haskell channel and the Haskell community at large will have one less friendly face who always enjoys helping new people learn.

    However, I wanted to just put some words down about his personal influence in my Haskell, Academic, and FOSS career.

    When I was a new Haskeller, a lot of things confused me. But the passion of people like Ertugrul to help me understand concepts that I found interesting late into the night was one of the things that really made it worth it.

    One of these lead to the creation of my first ever Haskell library, auto. auto is basically literally a direct translation of one of our conversations (and somewhat of a derivative of his own library netwire), and throughout the entire implementation process he was open to the many questions I had. And some of the features of the library (including implicit serialization) were directly his innovations put into practice.

    In a slightly different context — as a new PhD student, I was told to follow wherever my curiosity lead me. One of those lines lead me to “comonadic” image processing, which was directly inspired by this unfinished article of his.

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  • A Purely Functional Typed Approach to Trainable Models (Part 3)

    Hi again! Today we’re going to jump straight into tying together the functional framework described in this series and see how it can give us some interesting insight, as well as wrapping it up by talking about the scaffolding needed to turn this all into a working system you can apply today.

    The name of the game is a purely functional typed approach to writing trainable models using differentiable programming. Be sure to check out Part 1 and Part 2 if you haven’t, because this is a direct continuation.

    My favorite part about this system really is how we have pretty much free reign over how we can combine and manipulate our models, since they are just functions. Combinators — a word I’m going to be using to mean higher-order functions that return functions — tie everything together so well. Some models we might have thought were standalone entities might just be derivable from other models using basic functional combinators. And the best part is that they’re never necessary; just helpful.

    Again, if you want to follow along, the source code for the written code in this module is available on github.

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