# Dealing with multiple dispatch \`Vararg\`s

**URL:** <https://discourse.julialang.org/t/dealing-with-multiple-dispatch-vararg-s/127935>\
**Category:** General Usage\
**Tags:** question\
**Created:** [April 10, 2025, 2:58pm UTC](https://discourse.julialang.org/t/dealing-with-multiple-dispatch-vararg-s/127935 "2025-04-10T14:58:54Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![MilesCranmer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/milescranmer/32/21070_2.png) [@MilesCranmer](https://discourse.julialang.org/u/MilesCranmer)\
**Post date:** [April 10, 2025, 2:58pm UTC](https://discourse.julialang.org/t/dealing-with-multiple-dispatch-vararg-s/127935/1 "2025-04-10T14:58:54Z")

</div>

Say that we have two types:

```julia
struct A end #= other library =#
struct B end #= my library =#

```

Now, let’s assume that in some other library there is a method:

```julia
foo(x, xs...) = 1
foo(x::A, xs::A...) = 1

```

I want to overload this method to return `2`, for situations where _ **any of** _ `(x, xs...)` is type `B`.

In theory, this should be valid in the method hierarchy, right? I just can’t figure out how to actually express it. Is there a way I can do this **without causing ambiguities or committing piracy**?

So far I have been doing

```julia
foo(x::B, xs...) = 2
foo(x, x2::B, xs...) = 2
foo(x, x2, x3::B, xs...) = 2

```

which correctly captures cases where any of the first three arguments is `B`:

```julia
julia> foo(B())
2

julia> foo(A(), B())
2

julia> foo(A(), A(), B())
2

```

But past that point, we hit the generic method and get `1`.

```julia
julia> foo(A(), A(), A(), B())
1

```

So what are my options, other than defining up to some max number of arguments? I tried

```julia
julia> foo(x, xs::Union{Any,B}...) = 2

```

but this appears to be piracy, and overwrites the original `f(x, xs...)` method.

Maybe this isn’t possible?

* * *

As to why I want this: in [BorrowChecker.jl](https://discourse.julialang.org/t/ann-borrowchecker-jl-a-borrow-checker-for-julia/127897), I wish to overload the method

```julia
Base.rand(rng::AbstractRNG, d::Integer, dims::Integer...)

```

for cases where _at least one_ of `(d, dims...)` is type `AllWrappers{<:Integer}` (this `AllWrappers` type alias is a union of `Borrowed` and `Owned` wrapper objects). I could define all the permutations up to some integer, but seems like there should be a better way. Or maybe the answer is simply “don’t do this”… Or use Cassette.jl (now broken, sadly)… but, I think it would be nice to be able to overload methods like this.

---

<div class="post-metadata">

**Author:** ![Mason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mason/32/2423_2.png) [@Mason](https://discourse.julialang.org/u/Mason)\
**Post date:** [April 10, 2025, 3:00pm UTC](https://discourse.julialang.org/t/dealing-with-multiple-dispatch-vararg-s/127935/2 "2025-04-10T15:00:53Z")

</div>

Unfortunately you can’t without resorting to traits or whatever (which doesn’t help you if you don’t own the function being overloaded). There’s an open issue for it here: [Dispatch on a type being anywhere in a signature · Issue #39878 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/39878)