# From generator (with Channel) to iterator

**URL:** <https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277>\
**Category:** General Usage\
**Created:** [April 11, 2018, 12:04pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277 "2018-04-11T12:04:59Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![FemtoTrader](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/femtotrader/32/309_2.png) [@FemtoTrader](https://discourse.julialang.org/u/FemtoTrader)\
**Post date:** [April 11, 2018, 12:04pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/1 "2018-04-11T12:04:59Z")

</div>

Hello,

[PyGen - python style generators - #17 by nsmith](https://discourse.julialang.org/t/pygen-python-style-generators/3451/17) and [Creating Generators](https://discourse.julialang.org/t/creating-generators/3962) introduce how to mimics Python generator with Julia Channel.

With Python, a generator can be used like an iterator with `next` function (or ` __next__ ` method)

```python
In [1]: def gen():
   ...: for i in range(10):
   ...: yield i
   ...:         

In [2]: for i in gen():
   ...: print(i)
   ...:     
0
1
2
3
4
5
6
7
8
9

In [3]: g = gen()

In [4]: next(g) # or g. __next__ ()
Out[4]: 0

In [5]: next(g)
Out[5]: 1

In [6]: next(g)
Out[6]: 2

In [7]: next(g)
Out[7]: 3

In [8]: next(g)
Out[8]: 4

In [9]: next(g)
Out[9]: 5

In [10]: next(g)
Out[10]: 6

In [11]: next(g)
Out[11]: 7

In [12]: next(g)
Out[12]: 8

In [13]: next(g)
Out[13]: 9

In [14]: next(g)
---------------------------------------------------------------------------
StopIteration Traceback (most recent call last)
<ipython-input-14-5f315c5de15b> in <module>()
----> 1 next(g)

StopIteration: 

```

I wonder if a Julia generator implemented using Channel can be used like an iterator (ie have a `next` function implementation)

```julia
julia> gen() = Channel(ctype=Int) do c
         for i in 0:10-1
           push!(c, i)
         end
       end
gen (generic function with 1 method)

julia> g = gen()

julia> for i in g
         println(i)
       end
0
1
2
3
4
5
6
7
8
9

```

Unfortunatelly it doesn’t work as expected:

```julia
julia> g = gen()

julia> state = start(g)
Base.ChannelIterState{Int64}(false, 0)

julia> i, state = next(g, state)
(0, Base.ChannelIterState{Int64}(false, 0))

julia> i, state = next(g, state)
(0, Base.ChannelIterState{Int64}(false, 0))

julia> i, state = next(g, state)
(0, Base.ChannelIterState{Int64}(false, 0))

```

Any idea why it’s behaving this way?

Kind regards

---

<div class="post-metadata">

**Author:** ![vvjn](https://avatars.discourse-cdn.com/v4/letter/v/5f9b8f/32.png) [@vvjn](https://discourse.julialang.org/u/vvjn)\
**Post date:** [April 11, 2018, 2:08pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/2 "2018-04-11T14:08:52Z")

</div>

If you look at the code ([https://github.com/JuliaLang/julia/blob/master/base/channels.jl](https://github.com/JuliaLang/julia/blob/master/base/channels.jl)), you will see that `done` is where the next value in the Channel is extracted. So before every `next` call, you have to call `done(g, state)`. Pretty sure `for` loops call `done` before every call to `next`.

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [April 11, 2018, 2:20pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/3 "2018-04-11T14:20:35Z")

</div>

Fixing this is precisely what the iteration protocol change mentioned [here](https://discourse.julialang.org/t/1-0-progress-status/9762) is about.

---

<div class="post-metadata">

**Author:** ![FemtoTrader](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/femtotrader/32/309_2.png) [@FemtoTrader](https://discourse.julialang.org/u/FemtoTrader)\
**Post date:** [April 11, 2018, 2:25pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/4 "2018-04-11T14:25:26Z")

</div>

You are right

```julia
julia> g = gen()
Channel{Int64}(sz_max:0,sz_curr:1)

julia> state = start(g)
Base.ChannelIterState{Int64}(false, 0)

julia> done(g, state)
false

julia> i, state = next(g, state)
(0, Base.ChannelIterState{Int64}(false, 0))

julia> done(g, state)
false

julia> i, state = next(g, state)
(1, Base.ChannelIterState{Int64}(false, 1))

julia> done(g, state)
false

julia> i, state = next(g, state)
(2, Base.ChannelIterState{Int64}(false, 2))

julia> done(g, state)
false

julia> i, state = next(g, state)
(3, Base.ChannelIterState{Int64}(false, 3))

julia> done(g, state)
false

julia> i, state = next(g, state)
(4, Base.ChannelIterState{Int64}(false, 4))

julia> i, state = next(g, state)
(4, Base.ChannelIterState{Int64}(false, 4))

julia> i, state = next(g, state)
(4, Base.ChannelIterState{Int64}(false, 4))

julia> i, state = next(g, state)
(4, Base.ChannelIterState{Int64}(false, 4))

julia> done(g, state)
false

julia> i, state = next(g, state)
(5, Base.ChannelIterState{Int64}(false, 5))

```

so if I understand correctly @StefanKarpinski with Julia 0.7 (or 1.0?), calling next (without done) will be enough, isn’t it?

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [April 11, 2018, 2:29pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/5 "2018-04-11T14:29:38Z")

</div>

Not yet, but once @keno merges his changes, instead of `start`, `next` and `done` there will only be `iterate`:

- `iterate(itr)` will replace `start(itr)`
- `iterate(itr, state)` will replace `done(itr, state)` and `next(itr, state)`:
  - if it returns `nothing` then the iterator is done;
  - if it returns a `(value, state)` pair then it’s not.

This allows iterators like channels to try getting a new value and decide _after_ trying whether they’re done or not.

---

<div class="post-metadata">

**Author:** ![FemtoTrader](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/femtotrader/32/309_2.png) [@FemtoTrader](https://discourse.julialang.org/u/FemtoTrader)\
**Post date:** [April 24, 2018, 7:07am UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/6 "2018-04-24T07:07:44Z")

</div>

I wonder if this discussion have ever been linked with one with Do While Loop such as [Redirecting to Google Groups](https://groups.google.com/forum/#!topic/julia-dev/RV3BNkXN80w) because this it typically a case where you need to run a conditional instruction after one loop.

---

<div class="post-metadata">

**Author:** ![jandehaan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jandehaan/32/6805_2.png) [@jandehaan](https://discourse.julialang.org/u/jandehaan)\
**Post date:** [April 24, 2018, 11:01pm UTC](https://discourse.julialang.org/t/from-generator-with-channel-to-iterator/10277/7 "2018-04-24T23:01:16Z")

</div>

I noticed that `Stateful` has recently been added to Base.Iterators (not available in v6.2.2 😐)  
It implements an iterator that knows about its own state.  
Question: what is the reason regular iterators externalize their state? Or, said differently, what would we lose if all iterators were Stateful and hide their state?