How to capture an index in closure without allocating (or workaround)?

General Usage Performance
5

Remembering that “Closures are poor man’s objects and objects are poor man’s closures” you can explicitly represent and pass in your closure:

mutable struct MyInner{T1, T2}
    i::T1
    res::T2
end

function (in::MyInner)(c1, c2)
    allCombi = Iterators.flatten(Iterators.product(c1, c2))
    for combi in allCombi
        in.i += 1
        in.res[in.i] = combi[1] > 5.0 ? nothing : 2.0
    end
    return
end

function testIt!(inner, v1, v2)
    function outer(c1)
        itrCombis(c2 -> inner(c1, c2), v2, Val(4))
        return
    end
    
    itrCombis(outer, v1, Val(4))
    return
end

function run()
    res = Vector{Union{Nothing,Float64}}(undef, 100000000)
    num = 5
    v1 = collect(1:num)
    v2 = collect(1:num)
    in = MyInner(0, res)
    testIt!(in, v1, v2)
    @time testIt!(in, v1, v2)
end

Don’t think its worth the effort, just to prevent 16 bytes of allocation.