args... and ; kwargs... is the splat operator, if you know Python, it's the same as *args and **kwargs:
You can find documentation here: What does the splat ... operator do?
When in method signature (collects arguments).
julia> function foo(pos_1, pos_2, opt_1 = :opt_1, args...; opt_kw1 = :opt_kw1, opt_kw2 = :opt_kw2, kwargs...) [pos_1, pos_2, opt_1, args, opt_kw1, opt_kw2, (kwargs,);] end foo (generic function with 2 methods)
This signature means:
pos_1 is the first required positional argument.pos_2 is the second required positional argument.opt_1 is an optional positional argument.args... are all the following positional arguments, collected in a tuple.
Notice how semi colon ; separates positional arguments from keyword arguments (order is not important in key word arguments):
opt_kw1 is an optional keyword argument.opt_kw2 is an optional keyword argument.kwargs... are all the following keyword arguments collected in an array of tuple (key, value) pairs.
julia> methods(foo) # 2 methods for generic function "foo": foo(pos_1, pos_2) at none:3 foo(pos_1, pos_2, opt_1, args...) at none:3
foo can be called like this:
Required arguments:
julia> foo(:pos_1, :pos_2) 7-element Array{Any,1}: :pos_1 # provided value :pos_2 # provided value :opt_1 # default value () # empty tuple :opt_kw1 # default value :opt_kw2 # default value (Any[],) # there are no kwargs
Optional positional and keyword arguments:
julia> foo(:pos_1, :pos_2, :OPT_1, :a, :b, :c, opt_kw2 = :OPT_KW2, kwarg1 = true, opt_kw1 = :OPT_KW1, kwarg2 = false) 7-element Array{Any,1}: :pos_1 :pos_2 :OPT_1 (:a,:b,:c) :OPT_KW1 :OPT_KW2 (Any[(:kwarg1,true),(:kwarg2,false)],)
Notice how the order in key word arguments is not relevant, also the semicolon ; is not needed when calling the function.
When in function call (spreads arguments).
Using collections for the positional arguments and assosiative collections with symbol keys for the keyword arguments:
julia> x, y, z = 1, 2, 3; julia> sum(x, y, z) ERROR: MethodError: `sum` has no method matching sum(::Int64, ::Int64, ::Int64) Closest candidates are: sum(::Union{Base.Func{1},Function}, ::AbstractArray{T,N}, ::Any) sum(::Union{Base.Func{1},DataType,Function}, ::Any) sum(::BitArray{N}, ::Any) ... julia> sum sum (generic function with 12 methods) julia> Base.sum(args...) = sum(args) sum (generic function with 13 methods) julia> sum(x, y, z) 6 julia> foo(x, y, z) = sum(x, y, z) foo (generic function with 1 method) julia> foo(x, y, z) 6 julia> foo([x, y, z]) ERROR: MethodError: `foo` has no method matching foo(::Array{Int64,1}) Closest candidates are: foo(::Any, ::Any, ::Any) julia> foo([x, y, z]...) 6
julia> foo(; x = 0, y = 0, z = 0) = sum(x, y, z) foo (generic function with 2 methods) julia> foo() 0 julia> foo(z = 3, x = 1, y = 2) 6 julia> foo(; Dict(:z => 3, :y => 2, :x => 1)) ERROR: TypeError: anonymous: in typeassert, expected Symbol, got Pair{Symbol,Int64} in anonymous at no file julia> foo(; Dict(:z => 3, :y => 2, :x => 1)...) 6
