Julia Language Defining an enumerated type


An enumerated type is a type that can hold one of a finite list of possible values. In Julia, enumerated types are typically called "enum types". For instance, one could use enum types to describe the seven days of the week, the twelve months of the year, the four suits of a standard 52-card deck, or other similar situations.

We can define enumerated types to model the suits and ranks of a standard 52-card deck. The @enum macro is used to define enum types.

@enum Suit ♣ ♦ ♥ ♠
@enum Rank ace=1 two three four five six seven eight nine ten jack queen king

This defines two types: Suit and Rank. We can check that the values are indeed of the expected types:

julia> ♦
♦::Suit = 1

julia> six
six::Rank = 6

Note that each suit and rank has been associated with a number. By default, this number starts at zero. So the second suit, diamonds, was assigned the number 1. In the case of Rank, it may make more sense to start the number at one. This was achieved by annotating the definition of ace with a =1 annotation.

Enumerated types come with a lot of functionality, such as equality (and indeed identity) and comparisons built in:

julia> seven === seven

julia> ten ≠ jack

julia> two < three

Like values of any other immutable type, values of enumerated types can also be hashed and stored in Dicts.

We can complete this example by defining a Card type that has a Rank and a Suit field:

immutable Card

and hence we can create cards with

julia> Card(three, ♣)
Card(three::Rank = 3,♣::Suit = 0)

But enumerated types also come with their own convert methods, so we can indeed simply do

julia> Card(7, ♠)
Card(seven::Rank = 7,♠::Suit = 3)

and since 7 can be directly converted to Rank, this constructor works out of the box.

We might wish to define syntactic sugar for constructing these cards; implicit multiplication provides a convenient way to do it. Define

julia> import Base.*

julia> r::Int * s::Suit = Card(r, s)
* (generic function with 156 methods)

and then

julia> 10♣
Card(ten::Rank = 10,♣::Suit = 0)

julia> 5♠
Card(five::Rank = 5,♠::Suit = 3)

once again taking advantage of the in-built convert functions.