Jakub Arnold

Fibonacci NumbersAug 8, 2015

The Fibonacci numbers are a well known recursive sequence, which is defined as followed

f[0] = 0
f[1] = 1
f[n] = f[n-1] + f[n-2]

The question is, how can we calculate them?

The first idea and probably most intuitive way is recursively. Why? Because the structure of the sequence itself is recursive, which means the implementation will be very similar to our definition.

I’ll chose JavaScript as the implementation language, simply because you can just open the developer console in your browser and paste in the snippets to see the results immediately...

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Parsing CSS with ParsecAug 10, 2014

This article is a small introduction to Parsec, the Haskell parser combinator library for Haskell. We’ll use it to parse simple CSS file such as the following.

.container h1 {
  color: rgba(255, 0, 0, 0.9);
  font-size: 24px;

First we need to figure out our data structure which will represent the syntax tree. Since this is just an introduction, we’ll go easy and ignore features like media queries.

In order to create the structure we need to figure out how to name things. We can look at the grammar definition for CSS 2.1 to figure out how...

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Lens Tutorial - Stab & Traversal (Part 2)Aug 6, 2014

In the first article in the series about lenses, we’ve looked at the motivation behind the lens library, and we also derived the basic type of Lens s a.

In this article we’ll go deeper and explain the reasoning beheind the more generic Lens s t a b type. We’ll also take a look at how we can get a multi focus lens using a Traversal.

Just to reiterate, here’s how looks the type we derived in the previous article.

type Lens s a = forall f. Functor f => (a -> f a) -> s -> f s

What we’ll do here is further generalize it so that we can change the...

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Foldable and TraversableJul 30, 2014

Before we can get into the more advanced topics on Lenses, it is important to really understand both Foldable and Traversable, which is the motivation behind this article.

Let’s begin with Foldable. Foldable represents structures which can be folded. What does that mean? Here are a few examples:

  • Calculating the sum of a list.
  • Calculating the product of a list.
  • Folding a tree to get a maximum value.

We can describe a fold as taking a structure and reducing it to a single result. That’s also why some languages have a reduce function instead...

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Building Monad Transformers - Part 1Jul 22, 2014

In this article we’ll focus on building our own monad transformers. We’ll start out with an example code and improve it by building a simple wrapper over IO (Maybe a).

The following example is really simple, but I’m sure you can imagine doing something similar in your own application. The findById method is there just to simulate a database query that might not find a result.

data User = User deriving Show

findById :: Int -> IO (Maybe User)
findById 1 = return $ Just User
findById _ = return Nothing

findUsers :: Int -> Int -> IO (Maybe (User

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Mutable State in HaskellJul 20, 2014

Haskell is a purely functional language, which means there are no side-effects and all variables are immutable. But as you probably know this isn’t completely true. All variables are indeed immutable, but there are ways to construct mutable references where we can change what the reference points to.

Without side effects we wouldn’t be able to do much, which is why Haskell gives us the IO monad. In a similar manner we have many ways to achieve mutable state in Haskell, let’s take a look at them:

  • IORef
  • STRef in the ST monad
  • MVar
  • TVar in Software...

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Lens Tutorial - Introduction (part 1)Jul 14, 2014

This article is the first in the upcoming series that aims to explain the Haskell’s lens library and the ideas behind it in an approachable way. Don’t worry if you’re new to Haskell, the only prerequisites here should be understanding of the Functor type class, and understanding how records and algebraic data types work in Haskell.

We won’t be using the lens library in this article yet. The API we’ll develop will be exactly the same, but for the sake of learning I’ll try to show you how everything works and why it works by re-implementing it...

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Using Phantom Types in Haskell for Extra Safety - Part 2Jul 10, 2014

I’ve received a lot of reactions to the previous blog post about Phantom Types over the past two days, which is why I’ve decided to summarize what I’ve learned in another blog post.

First, here’s a summarized problem from the previous post. We have a Message which can be either PlainText or Encrypted. We’ve used Phantom Types to enforce this in the type system:

data Message a = Message String

data PlainText
data Encrypted

send :: Message Encrypted -> IO ()
encrypt :: Message PlainText -> Message Encrypted
decrypt :: Message Encrypted -> Message

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Using Phantom Types for Extra SafetyJul 8, 2014

If you’ve been programming in a dynamic language, you’ve probably heard that type systems can catch more errors before your application even gets run. The more powerful the type system is, the more you can express in it. And because we’re talking about Haskell, we have a great number of tools at our disposal when trying to express things in terms of the types.

Why is this important? Sometimes a function has an expectation about the value that it’s receiving. In most imperative languages those expectations are implicit and up to the programmer...

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Evil Mode: How I Switched From VIM to EmacsJun 23, 2014

I’ve been a long time VIM user. I use it every day for all of my work and side projects, writing blog posts, writing other content, sometimes even for writing emails if the text is long enough. VIM is like my home and I’m deeply in love with it.

The problem is that VIM is a horrible IDE. It’s an amazing and super productive editor, but it really sucks at doing IDE-like things. Now you might be thinking I’m a noob who needs to click on good looking buttons in RubyMine to get things done. No, that’s not what I mean by IDE … let me explain.


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