Frederico Honório fredshonorio

Type Safe JSON Decoding in Elm

The power of a Static Typed language can seem magical at first. But the goal here is to take a tiny peak behind that curtain.

Elm's implementation of JSON parsing is type safe and how it achieves that can seem like a mystery. Even though I got the code to work, it took me a while to fully understand how it works.

I'm writing it down here for 2 reasons. To help others gain a greater understanding of Types and so I don't forget what I learned.

Word of Caution

Default Values in Elm

Using Maybe

In Elm, optional values are handled with Maybe. The following functions show a simple implementation of optional parameters:

combineOpts : Maybe String -> Maybe String -> String -> String -> String
combineOpts = prefix suffix s1 s2 =

Should be work with 0.18

Destructuring(or pattern matching) is a way used to extract data from a data structure(tuple, list, record) that mirros the construction. Compare to other languages, Elm support much less destructuring but let's see what it got !

Tuple

myTuple = ("A", "B", "C")
myNestedTuple = ("A", "B", "C", ("X", "Y", "Z"))

Reading & Understanding Types: Exercises to Level Up!

A type is a set of values. A value stores information at runtime in computer memory (such as a certain integer, a certain list of strings, etc.).

Monomorphic Function Types

In many languages, functions are values (Haskell, PureScript, Javascript)! Or at least, you can pretend they are (Scala, Java).

agnoster.zsh-theme

A ZSH theme optimized for people who use:

Solarized
Git
Unicode-compatible fonts and terminals (I use iTerm2 + Menlo)

For Mac users, I highly recommend iTerm 2 + Solarized Dark

Gradle Dependency Resolution

Normal Gradle behavior

The default behavior of Gradle to pick the newest version also applies if a lower version has been declared locally, but another dependency transitively pulls in a newer version. This is in contrast with Maven, where a locally declared version will always win.

For example, if your build.gradle specifies the dependency org.springframework:spring-tx:3.2.3.RELEASE, and another dependency declares 4.0.5.RELEASE as a transitive dependency, then 4.0.5.RELEASE will take precedence:

dependencies {
    compile("org.springframework.data:spring-data-hadoop:2.0.0.RELEASE")
    compile("org.springframework:spring-tx:3.2.3.RELEASE")

// will select org.springframework:spring-tx:4.0.5.RELEASE

Helpful links:

	README.txt - DIY Scala Enumeration
	Copyright (C) 2014-2016 Jim O'Flaherty

	Overview:
	Provide in Scala the closest equivalent to Java Enum
	- includes decorating each declared Enum member with extended information
	- guarantees pattern matching exhaustiveness checking
	- this is not available with scala.Enumeration

	ScalaOlio library (GPLv3) which contains more up-to-date versions of both `org.scalaolio.util.Enumeration` and `org.scalaolio.util.EnumerationDecorated`:

	/**
	* ```
	* Does JDK8's Optional class satisfy the Monad laws?
	* =================================================
	* 1. Left identity: true
	* 2. Right identity: true
	* 3. Associativity: true
	*
	* Yes, it does.
	* ```