# Expressing "at least one of this type" on Julia master

**URL:** https://discourse.julialang.org/t/expressing-at-least-one-of-this-type-on-julia-master/8624
**Category:** General Usage
**Tags:** question
**Created:** [January 26, 2018, 7:33pm UTC](https://discourse.julialang.org/t/expressing-at-least-one-of-this-type-on-julia-master/8624 "2018-01-26T19:33:20Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [January 26, 2018, 7:33pm UTC](https://discourse.julialang.org/t/expressing-at-least-one-of-this-type-on-julia-master/8624/1 "2018-01-26T19:33:20Z")

</div>

[https://github.com/JuliaLang/julia/pull/23117](https://github.com/JuliaLang/julia/pull/23117) removed a hack that was used in a number of places to express the concept of a set of arguments, at least one of which is of a particular type. For example, if I have a custom array type and I wanted to overload `Base.foreach` when at least one argument is of my custom type, I could do:

```julia
struct VectorLike{V <: AbstractVector}
	x::V
end

_getvector(x::VectorLike) = x.x
_getvector(x::AbstractVector) = x

function Base.foreach(f, vs::Union{AbstractVector, VectorLike{V}}...) where {V}
	println("custom foreach with inner type $V")
	foreach(f, _getvector.(vs)...)
end

```

And I would get the following on Julia v0.6:

```julia
julia> foreach((x, y) -> println(x, y), [1,2,3], [4,5,6])
14
25
36

julia> foreach((x, y) -> println(x, y), VectorLike([1,2,3]), [4,5,6])
custom foreach with inner type Array{Int64,1}
14
25
36

```

However, on Julia master, the `Base.foreach` signature above now matches any number of AbstractVectors, regardless of whether any of them is of my `VectorLike` type:

```julia
julia> foreach((x, y) -> println(x, y), [1,2,3], [4,5,6])
ERROR: UndefVarError: V not defined
Stacktrace:
 [1] foreach(::Function, ::Array{Int64,1}, ::Array{Int64,1}, ::Vararg{Array{Int64,1},N} where N) at /home/rdeits/Projects/jump-containers/types.jl:9
 [2] top-level scope

```

I realize that this was exactly the intent of [https://github.com/JuliaLang/julia/pull/23117](https://github.com/JuliaLang/julia/pull/23117) , but I’m wondering how to express this desired behavior at all on Julia v0.7+. Any suggestions?

One idea I had would be to define an `@generated` `Base.foreach` that accepts any number of `AbstractVectors` and then checks in the generator if any of them is a `VectorLike`. But this would make _all_ `foreach` calls pass through my new `@generated` function, which is clearly suboptimal (and seems like it would conflict with other packages trying to do the same thing).

Another option is to generate all of the matching signatures up to some number of arguments, but that requires generating O(N^2) different methods (and choosing some max number of arguments N) which also seems suboptimal.

---

<div class="post-metadata">

### Author: ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)
#### Post date: [January 26, 2018, 8:47pm UTC](https://discourse.julialang.org/t/expressing-at-least-one-of-this-type-on-julia-master/8624/2 "2018-01-26T20:47:12Z")

</div>

See also [https://github.com/JuliaLang/julia/issues/21026#issuecomment-322878914](https://github.com/JuliaLang/julia/issues/21026#issuecomment-322878914).

Broadcast is able to handle this kind of thing using `BroadcastStyle` and binary broadcasting rules, [https://docs.julialang.org/en/latest/manual/interfaces/#writing-binary-broadcasting-rules-1](https://docs.julialang.org/en/latest/manual/interfaces/#writing-binary-broadcasting-rules-1). The machinery would need to be set up in Base to dispatch on the equivalent of `BroadcastStyle` for `foreach`. You could just overwrite the Base methods to do so.