Enums

Syntax#

@enum EnumType val=1 val val
:symbol

Remarks#

It is sometimes useful to have enumerated types where each instance is of a different type (often a singleton immutable type); this can be important for type stability. Traits are typically implemented with this paradigm. However, this results in additional compile-time overhead.

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
true

julia> ten ≠ jack
true

julia> two < three
true

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
    rank::Rank
    suit::Suit
end

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.

Using symbols as lightweight enums

Although the @enum macro is quite useful for most use cases, it can be excessive in some use cases. Disadvantages of @enum include:

It creates a new type
It is a little harder to extend
It comes with functionality such as conversion, enumeration, and comparison, which may be superfluous in some applications

In cases where a lighter-weight alternative is desired, the Symbol type can be used. Symbols are interned strings; they represent sequences of characters, much like strings do, but they are uniquely associated with numbers. This unique association enables fast symbol equality comparison.

We may again implement a Card type, this time using Symbol fields:

const ranks = Set([:ace, :two, :three, :four, :five, :six, :seven, :eight, :nine,
                   :ten, :jack, :queen, :king])
const suits = Set([:♣, :♦, :♥, :♠])
immutable Card
    rank::Symbol
    suit::Symbol
    function Card(r::Symbol, s::Symbol)
        r in ranks || throw(ArgumentError("invalid rank: $r"))
        s in suits || throw(ArgumentError("invalid suit: $s"))
        new(r, s)
    end
end

We implement the inner constructor to check for any incorrect values passed to the constructor. Unlike in the example using @enum types, Symbols can contain any string, and so we must be careful about what kinds of Symbols we accept. Note here the use of the short-circuit conditional operators.

Now we can construct Card objects like we expect:

julia> Card(:ace, :♦)
Card(:ace,:♦)

julia> Card(:nine, :♠)
Card(:nine,:♠)

julia> Card(:eleven, :♠)
ERROR: ArgumentError: invalid rank: eleven
 in Card(::Symbol, ::Symbol) at ./REPL[17]:5

julia> Card(:king, :X)
ERROR: ArgumentError: invalid suit: X
 in Card(::Symbol, ::Symbol) at ./REPL[17]:6

A major benefit of Symbols is their runtime extensibility. If at runtime, we wish to accept (for example) :eleven as a new rank, it suffices to simply run push!(ranks, :eleven). Such runtime extensibility is not possible with @enum types.