fast_instance_iteration.jl

using BenchmarkTools

macro fast_instance_iteration(_instance::Symbol)
    local outer_instance = getfield(@__MODULE__, _instance)
    local dispatch_on_instance = Val{outer_instance}
    local base_type = typeof(outer_instance)
    local field_values = [getfield(outer_instance, field) for field in fieldnames(base_type)]
    local num_fields = length(field_values)
    local id = rand()

    local ex = Expr(:block)
    ex.args = [
        quote
        Base.iterate(instance::$base_type) = iterate(instance, Val{instance}())
        Base.iterate(instance::$base_type, don::$dispatch_on_instance) = iterate(instance, (Val{1}(), Val{$id}()))
        end |> esc
    ]

    for i in 1:num_fields
        push!(ex.args,
              quote
              Base.iterate(instance::$base_type, ::Tuple{Val{$i}, Val{$id}}) = ($(field_values[i]), (Val{$(i+1)}(), Val{$id}()))
              end |> esc
              )
    end

    push!(ex.args, quote Base.iterate(instance::$base_type, ::Tuple{Val{$(num_fields+1)}, Val{$id}}) = nothing end |> esc)
    return ex

end


macro fast_instance_iteration2(_instance::Symbol)
    local outer_instance = getfield(@__MODULE__, _instance)
    local dispatch_on_instance = typeof(Val(outer_instance))
    local base_type = typeof(outer_instance)
    local field_values = [getfield(outer_instance, field) for field in fieldnames(base_type)]
    local num_fields = length(field_values)

    quote
        Base.iterate(instance::$base_type, idx=1) = iterate(Val(instance), idx)
        Base.iterate(instance::$dispatch_on_instance, idx=1) = idx > $num_fields ? nothing : ($field_values[idx], idx+1)
    end |> esc
end

struct M
    a::Int
    b::Int
    c::Int
    d::Int
    e::Int
    f::Int
    g::Int
    h::Int
    i::Int
    j::Int
    k::Int
    l::Int
end

m = M(10,20,30,40,50,60,70,80,90,100,110,120)
m2 = M(1:2:24...)

@fast_instance_iteration m
@fast_instance_iteration m2

for i in m
    print(i, "/")
end

for i in m2
    print(i, "/")
end

function test()
    for i in m
    end
    for i in m2
    end
end

@btime test()

@fast_instance_iteration2 m
@fast_instance_iteration2 m2

for i in m
    print(i, "/")
end

for i in m2
    print(i, "/")
end

@btime test()