Multiple dispatch and hierarchical data types in Julia

By Cory Simon

In our research group, we write our codes to conduct molecular simulations in the Julia programming language. We choose Julia because it is a high-level, dynamic language, but, owing to its design and just-in-time compiler, Julia approaches the speed of C. Compared to writing code in C, Julia enables us to write similarly fast code with fewer lines and lower complexity.

In this post, we give examples to show two powerful features of Julia: hierarchical data types and multiple dispatch. Let’s use wine as an example. :wine_glass:

Hierarchical data types

In Julia, we can define an abstract type, GlassOfWine, for a glass of wine:

abstract type GlassOfWine end

Then we can define two subtypes of glasses of wine, one for Pinot Noir and one for Grenache. Let’s say all we care about is the region the wine is from and the volume we have in our glass. But, for Pinot Noir, we’re also concerned about the price.

type PinotNoir<:GlassOfWine
    volume::Float64 # ounces
    price::Float64 # $

type Grenache<:GlassOfWine
    volume::Float64 # ounces

The <:GlassOfWine part declares that PinotNoir and Grenache are subtypes of GlassOfwine. You can confirm by subtypes(GlassOfWine) in Julia. Or supertype(PinotNoir) and supertype(Grenache), which will return GlassOfWine.

With this abstract type GlassOfWine, we can write one function, sip!, that operates on both types of wine glasses.

function sip!(glass_of_wine::GlassOfWine)
    glass_of_wine.volume -= 0.1

The function sip! (the exclamation signifies that we are modifying the argument) operates on glasses of both Pinot Noir and Grenache.

wg = PinotNoir("Oregon", 5.0, 25.0)
wg.volume # 4.9

wg = Grenache("Spain", 5.0)
wg.volume # 4.9

That’s a simple example of a type hierarchy in Julia.

Multiple dispatch

In Julia, we can write a function with the same name that behaves differently depending on the data type passed to it. For example, guess_region will guess the region a GlassOfWine is from, but will guess a different region depending on whether it is a PinotNoir or a Grenache.

function guess_region(glass_of_wine::PinotNoir)
    println("Is this glass of wine from Oregon?")
    glass_of_wine.region == "Oregon" ? println("Yes") : println("No")

function guess_region(glass_of_wine::Grenache)
    println("Is this glass of wine from Spain?")
    glass_of_wine.region == "Spain" ? println("Yes") : println("No")

Now if we have two glasses of wine, each from Oregon, but one Pinot Noir and one Grenache, guess_region will behave differently for each glass:

pn = PinotNoir("Oregon", 5.0, 25.0)
g = Grenache("Oregon", 5.0)

# Is this glass of wine from Oregon?
# Yes
# Is this glass of wine from Spain?
# No