julia> fun(x::Integer) = 1
fun (generic function with 1 method)
julia> fun(x::Integer; y::Array) = 1
fun (generic function with 1 method)
julia> fun(x::Integer, y::Array) = 1
fun (generic function with 2 methods)
Why doesn’t the 2nd definition create another method?
julia> fun(x::Integer; y::Array= nothing) = 1
fun (generic function with 1 method)
But in my situation, I am expanding another function’s methods, and the function has fun(x::Integer) = 1 originally, and I don’t want to overwrite that method.
In the Slack group, It is pointed out that keyword arguments never participate in dispatch, and so there is no way other than redefining the original methods.
opened 07:55AM - 30 Dec 14 UTC
bug
doc
decision
types and dispatch
keyword arguments
So this is an issue that I find after figuring out how the keyword arguments are… currently implemented in Julia. It will probably not be an issue anymore if #2773 is implemented.
The document on methods says
> Methods are dispatched based only on positional arguments, with keyword arguments processed after the matching method is identified.
However, method dispatch actually behaves differently when doing a function call with or without keyword arguments. i.e.
``` julia
julia> function f(::Integer)
2
end
f (generic function with 1 method)
julia> function f(::Number; kw...)
1
end
f (generic function with 2 methods)
julia> f(1)
2
julia> f(1; a = 2)
1
```
What happens here is that `f.env.kwsorter` only has one method defined and therefore when calling with keyword argument, `f(::Integer)` does not participate in method dispatch.
IMHO, there are several possible ways to fix it,
1. Fix the document to include this behavior. This should be the easiest fix but will probably make the whole keyword argument/optional argument/multiple dispatch more confusing especially for someone who does not know how it all works behind the scene. (It's already quite confusing/surprising that anonymous function does not support keyword argument for someone (like me) that expects python-like keyword argument implementation.)
2. Having an entry (that just throw an error) in `env.kwsorter` even for methods that does not take keyword arguments. This can also avoid the following confusing abuse of overriding method
``` julia
julia> function f(::Number; kw...)
1
end
f (generic function with 1 method)
julia> function f(::Number)
2
end
f (generic function with 1 method)
julia> f(1)
2
julia> f(1; a = 2)
1
```
This is probably the easiest way to fix the code and is consistent with the best long term behavior.
3. Fix #2773 and let the method themselves handle keyword argument dispatch. Since #2773 is on `1.0` milestone, hopefully this will be eventually be implemented.
If I type it to Int then the method is overwritten. But the point is that the second method should have a different signature in the positional parameter space. I chose Any but equally well I could have written e.g.:
f(x::Integer; kwarg) = "integer"
and the same problem would arise.
For a reference: this is not an artificial question but in DataFrames.jl we actually have methods which for “wider” (something like catch-all) method signature take keyword arguments but for narrower signatures di…
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