Different ways to declare Enum datatype in Julia-Lang

Question

Is using @enum the only way to declare Julia Enum datatype? If so why?

Answer 1

It's the only (easy) way, yes. The answer, as often (or, rather, always) in Julia, can be found by looking at the source code. This can be a bit scary at first, but you get used to it after a while!

Normally to create an object of a given type, you call the type's constructor. So you might expect to be able to do

Enum(...)

and create an object of type Enum.

In this case, however, Enum is an abstract type, so you cannot do that.

What does @enum do, then? The example from the manual is

julia> @enum FRUIT apple=1 orange=2 kiwi=3

This actually creates a completely new type, called FRUIT, that is a subtype of Enum, and objects of that type called apple, orange and kiwi, which are converted to those numbers by calling Int(apple) etc. This is done by generating the Julia code to do so, inside the macro.

In principle, you could, yourself, do all the work that the macro does, but the macro is there to make our life easier!

Edit: Since Julia 0.7, enums can be defined using @enum macros as you mentionned but can also be used with a begin block:

julia> @enum Fruit begin
           apple = 1
           orange = 2
           kiwi = 3
       end

julia> Fruit
Enum Fruit:
apple = 1
orange = 2
kiwi = 3

julia> apple
apple::Fruit = 1

julia> orange
orange::Fruit = 2

julia> kiwi
kiwi::Fruit = 3

julia> Int(orange)
2

julia> string(orange)
"orange"

Enums can also be defined with this begin block without specifying values (in such a case values are starting with 0, not 1)

julia> @enum Fruit begin
           apple
           orange
           kiwi
       end

julia> Fruit
Enum Fruit:
apple = 0
orange = 1
kiwi = 2