Generator?

generator_test.vim

function! Generator()
    function! Inner1(value) closure
        let i = a:value
        function! Inner2(value) closure
            let i += a:value
            function! Inner3(value) closure
                let i += a:value
                function! Inner4(value) closure
                    let i += a:value
                    return {'value': i, 'done': v:true}
                endfunction
                return {'value': i, 'done': v:false}
            endfunction
            return {'value': i, 'done': v:false}
        endfunction
        return {'value': i, 'done': v:false}
    endfunction

    let counter = 0
    function! Call(value) closure
        let counter += 1
        return Inner{counter}(a:value)
    endfunction
    return {'next': {value -> Call(value)}}
endfunction

let g = Generator()
echo g.next(1)
" -> {'done': v:false, 'next': 1}
echo g.next(2)
" -> {'done': v:false, 'next': 3}
echo g.next(3)
" -> {'done': v:false, 'next': 6}
echo g.next(4)
" -> {'done': v:true, 'next': 10}