Re: Hi, I am a little confused about stateless iterator.

[Hao Wu <wuhao.wise@...>]

On Mon, Dec 16, 2013 at 12:00 AM, zhuxiaohu <zxhfirefox@hotmail.com> wrote:
> Hi all,
>       I don't understand the different between stateless iterator and
> stateful iterator?
>
>        I know the iterator factory function return 3 values, the iter
> function, the invariant state, the initial value for control variable. Does
> the stateless iterator means that the iter function only use the invariant
> state and initial value for control variable to calculate the next value ?
> there is not any other local variable in iterator factory function (which
> will be the variables in closures ?
>
>        for the exercise 7.1 in Programming in Lua, does the follow code is
> correct answer ?
>
> --stateful
> --[[
> function fromto(n,m)
> local r = m - n
> return function(min, i)
> if i < r then
> return min + i
> else
> return nil
> end
> end,n,0
> end
> --]]
>
> --stateless
> local function iter(m,n)
> if n + 1 < m then
> return n + 1
> else
> return nil
> end
> end
>
> function fromto(n,m)
> return iter, m , n-1
> end
>
>
>
>
> for i in fromto(1,10) do
> print(i)
> end
>
>
> Thanks.
>

What you stated here looks correct to me. Stateless iterator should be
a pure function