Function Overloading in Julia - Keyword Argument

Question

Suppose a function fun() with keywords is overloaded like this:

function fun(; a::Real)
    return a
end

function fun(; a::Vector)
    return a
end

When executing the following code, I get a type error: TypeError: in keyword argument a, expected Vector, got a value of type Int64:

fun(1) # TypeError
fun([1, 2])

But when the function fun() gets normal arguments, i.e.:

function fun(a::Real)
    return a
end

function fun(a::Vector)
    return a
end

Then executing of this code snippet here:

fun(1) # -> 1
fun([1, 2]) # -> [1, 2]

works.

Why does the second example work, but the first does not?

Answer 1

Keyword arguments are not overloadable. In other words, they do not participate in method dispatch---only positional arguments participate in dispatch. When you define the second method, f(; a::Vector), it overwrites the first method:

julia> function fun(; a::Real)
           return a
       end
fun (generic function with 1 method)

julia> function fun(; a::Vector)
           return a
       end
fun (generic function with 1 method)

julia> methods(fun)
# 1 method for generic function "fun":
[1] fun(; a) in Main at REPL[2]:1