Dispatch on keywords

[Cyan]

I wrote these two functions

function test(;e::Float64)
    return e^2
end
function  test(;e::Int)
    return e*2
end

I obtain the right answer when test(1.) or test(1), but the test(e=1.) returns an error of:

TypeError: in keyword argument e, expected Int64, got a value of type Float64

What is wrong with the test(e=1.) ?

[jakobnissen]

Keyword arguments do not participate in dispatch. So, when you define the second method, the first one is overwritten (as it has the same signature, disregarding keyword arguments)

[giordano]

These aren’t two functions: you’re overwriting the only method of the single function test because keyword arguments don’t participate to dispatch.

[FerreolS]

If you want to dispatch on keyword, you can use some kind of barrier function :

test(;e) = _test(e)

function _test(e::Float64)
    return e^2
end
function  _test(e::Int)
    return e*2
end

julia> test(e=1)
2

julia> test(e=1.)
1.0

[danielmatz]

There’s also the very nice KeywordDispatch.jl package.

[matthias314]

@jakobnissen

Keyword arguments do not participate in dispatch.

Is this really true? The following 10-year old issue still applies to Julia 1.11:

Keyword arguments affect methods dispatch

[jakobnissen]

Yes, this is a bug.