Efficient flatten

efficient_flatten.jl

function Base.iterate(mf::MyFlatten{I,T}, state=nothing) where {I,T}
       if state===nothing
       val = iterate(mf.a)
       val === nothing && return nothing
       itr, s = val
       res = iterate(itr)
       else
       itr, s, inner_s = state
       res = iterate(itr, inner_s)
       end
       if res === nothing
       res2 = iterate(mf.a, s)
       res2 === nothing && return nothing
       itr, s = res2
       res3 = iterate(itr)
       res3 === nothing && return nothing
       return res3[1], (itr, s, res3[2])
       else
       return res[1], (itr, s, res[2])
       end
       end
       
Base.eltype(mf :: MyFlatten{I,T}) where {I,T} = T

Base.IteratorSize(::T) where T<:MyFlatten = Base.SizeUnknown()

struct MyFlatten{I,T}
       a::I
end