Check if a value is in an enum

scelles · December 3, 2019, 7:12pm

Hello,

I have an enum like

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

I can check that apple is a Fruit

julia> apple isa Fruit
true

But can’t check if 3 is an “allowed” value (ie a value which is used by this enum)

julia> 4 isa Fruit
false

julia> 3 isa Fruit
false

I tried 3 in Fruit or 3 in keys(Fruit) but enum are not iterable.

I haven’t dive into Enum internals… but maybe someone here can help.

(and docstring could probably be improved)

github.com

JuliaLang/julia/blob/master/base/Enums.jl#L95


      
          @noinline enum_argument_error(typename, x) = throw(ArgumentError(string("invalid value for Enum $(typename): $x")))
          
          
"""
              @enum EnumName[::BaseType] value1[=x] value2[=y]
          
          
Create an `Enum{BaseType}` subtype with name `EnumName` and enum member values of
          `value1` and `value2` with optional assigned values of `x` and `y`, respectively.
          `EnumName` can be used just like other types and enum member values as regular values, such as
          
          
# Examples
          ```jldoctest fruitenum
          julia> @enum Fruit apple=1 orange=2 kiwi=3
          
          
julia> f(x::Fruit) = "I'm a Fruit with value: \$(Int(x))"
          f (generic function with 1 method)
          
          
julia> f(apple)
          "I'm a Fruit with value: 1"
          
          
julia> Fruit(1)
          apple::Fruit = 1

Kind regards

stevengj · December 3, 2019, 7:57pm

You could use the instances function.

For example, you can define

const fruitvals = Set(Int(f) for f in instances(Fruit))

and then do

julia> 3 in fruitvals
true

julia> 17 in fruitvals
false

kristoffer.carlsson · December 3, 2019, 7:58pm

If you need something generic, the only real way I can see is the ugly

julia> function ismember(enum::Type{T}, v::Int) where T<:Enum
           try
               T(v)
               return true
           catch
               return false
           end
       end
ismember (generic function with 1 method)

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

julia> ismember(Fruit, 1)
true

julia> ismember(Fruit, 4)
false

The list of available values (https://github.com/JuliaLang/julia/blob/29b47b8a10397324a820717af655cb0ee1d7f023/base/Enums.jl#L133) is not easily available (edit: nvm, see below).

stevengj · December 3, 2019, 8:02pm

You can also use the undocumented internal function Base.Enums.namemap:

julia> 4 in keys(Base.Enums.namemap(Fruit))
false

and there are also the typemin and typemax functions to get the minimum and maximum allowed values:

julia> Int.((typemin(Fruit), typemax(Fruit)))
(1, 3)