Variant like in C++ for Julia

user_17818896 0
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How can I use use something like variant (which is in C++) in Julia?

In C++, I can do something like this

variant<int, float> value;

How can I do the same thing in Julia for a parameter of a function?

function func(??)
    # something
end

Sorry if it does not convey what I want to do.

julia variant
2 Answers

You can use union types to do the same.

function func(x::Union{Int, AbstractFloat})
    x + 1
end

Note that C++ std::variant is a tagged union, so you can always ask "which" variant it is, whereas a Julia union is a proper set union. Pragmatically, that means that in std::variant<int, int>, you can distinguish between a "left int" and a "right int", but a Union{Int, Int} really is just an Int. There's no way to tell which "side" it came from. To emulate the exact tagged union behavior, you'll need to make some custom structs to represent each case.

struct LeftInt
    value :: Int
end

struct RightInt
    value :: Int
end

function func(x::Union{LeftInt, RightInt})
    # ... Check whether x is a LeftInt or not ...
end

but, in Julia, you often don't need this, and if you do then it's generally smarter to work with generic functions to do your dispatching anyway.

user_22886590

In Julia, you can just write

function func(value)
    # something
end

and the compiler will then automatically compile for you separate type-specific method of this untyped function whenever you call it with another type.

In Julia, unlike in C++, you usually don't provide a parameter type at all, and you instead let the compiler figure out the types for you. Any parameter types that you provide are just assertions, or help to dispatch to the right method in case you want to provide different implementations for different types yourself.

user_19906890
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