# Exception Control in Iteration over Channels, Latest Version
**URL:** https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651
**Category:** General Usage
**Tags:** question
**Created:** [August 17, 2025, 1:00pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651 "2025-08-17T13:00:50Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@mraic1](https://discourse.julialang.org/u/mraic1)
#### Post date: [August 17, 2025, 1:00pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/1 "2025-08-17T13:00:50Z")
Dear all,
I got intrigued when I realized that exceptions can be silenced when iterating over a channel. However, the source code on GitHub indicates that exceptions should actually be rethrown. Then I realized that the file `Channels.jl` on GitHub is actually different from its counterpart on my computer, though I should have the latest version of Julia.
Now to details. Here is my example along with version details:
```julia-auto
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.11.6 (2025-07-09)
_/ |\ __'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |
julia> ch = Channel() do c
for x in 3:-1:-2
put!(c, sqrt(x))
end
end
Channel{Any}(0) (1 item available)
julia> for y in ch
println(y)
end
1.7320508075688772
1.4142135623730951
1.0
0.0
```
As one can see, no error is reported: when `sqrt(-1)` is to be computed, the iteration ends and the error is silenced.
However, the definition of the method of `iterate()` for channels in the file `channels.jl` on [GitHub](https://github.com/JuliaLang/julia/blob/master/base/channels.jl) reads:
```julia-auto
function iterate(c::Channel, state=nothing)
if isopen(c) || isready(c)
try
return (take!(c), nothing)
catch e
if isa(e, InvalidStateException) && e.state === :closed
return nothing
else
rethrow()
end
end
else
# If the channel was closed with an exception, it needs to be thrown
if (@atomic :acquire c.state) === :closed
e = c.excp
if isa(e, InvalidStateException) && e.state === :closed
nothing
else
throw(e)
end
end
return nothing
end
end
```
This code _does_ capture the error: when I defined a new function with that method, an error was reported. Then I realized that the file `channels.jl` on my computer is different. The underlying method reads:
```julia-auto
function iterate(c::Channel, state=nothing)
if isopen(c) || isready(c)
try
return (take!(c), nothing)
catch e
if isa(e, InvalidStateException) && e.state === :closed
return nothing
else
rethrow()
end
end
else
return nothing
end
end
```
This code silences possible exceptions, as displayed above.
However, I am wondering why the files are different. I have run `juliaup update`. The date of the file `channels.jl` on my computer is July 9, 2025, which matches the Julia version date. Is the file on GitHub (pointed by the official Julia documentation) actually a future version and is to appear on my computer after some time? Or is it, less likely, a dated version, so that exceptions are now _no longer_ captured in this situation? This is important for me to know: will be in future safe simply to use `Channel() do ... end` or should one create a task and a channel separately, bind the channel to the task, schedule the task and finally call `fetch()` to capture possible exceptions?
Many thanks for the answer in advance!
---
### Author:  [@sgaure](https://discourse.julialang.org/u/sgaure)
#### Post date: [August 17, 2025, 2:08pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/2 "2025-08-17T14:08:20Z")
The throwing `iterate` is in the master branch on github. The 1.11.6 source does not have it. It came with commit de1dba2c, Jan 13, 2025 in [Check channel state on iterate by jakobnissen · Pull Request #52981 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/52981)
---
### Author:  [@simsurace](https://discourse.julialang.org/u/simsurace)
#### Post date: [August 17, 2025, 3:45pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/3 "2025-08-17T15:45:46Z")
The channel creates a task. One can put an error monitor on the task in the scope where the channel is defined. The docstring of channel explains how to get the task reference.
---
### Author:  [@mraic1](https://discourse.julialang.org/u/mraic1)
#### Post date: [August 17, 2025, 4:08pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/4 "2025-08-17T16:08:56Z")
Hmm, so you are saying that the link to GitHub on the official Julia language page does not point to the branch associated to the current release. Indeed a bit surprising to me, though I am aware that I am not very familiar with the culture here. Nevertheless, there is of course a branch associated to the current release, so that you may wish to consider adding a link to that branch to the official Julia language page.
If I have noticed correctly, the version of `iterate` for channels in Julia 1.12 coincides with the version in the master branch. You have probably figured out that this is my desired version. Any idea when 1.12 will replace the current release?
Thanks a lot and all the best!
---
### Author:  [@mraic1](https://discourse.julialang.org/u/mraic1)
#### Post date: [August 17, 2025, 4:20pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/5 "2025-08-17T16:20:34Z")
Yes indeed, I have noticed how to get the task reference, but my impression is that references are not very usual in Julia. Am I right? Therefore, if the task is necessary, I prefer the longer version with creating it separately and binding the channel to it. In my opinion, the code is more readable this way. Nevertheless, thank you for your answer!
---
### Author:  [@sgaure](https://discourse.julialang.org/u/sgaure)
#### Post date: [August 17, 2025, 4:37pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/6 "2025-08-17T16:37:32Z")
1.12 is already in rc1, so I suppose some weeks until release. I don’t think there’s a fixed schedule, There are still some unfixed issues, but they might choose to postpone some of them. [GitHub · Where software is built](https://github.com/JuliaLang/julia/milestone/47).
---
### Author:  [@nsajko](https://discourse.julialang.org/u/nsajko)
#### Post date: [August 17, 2025, 4:49pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/7 "2025-08-17T16:49:42Z")
> [@mraic1](#):
>
> Hmm, so you are saying that the link to GitHub on the official Julia language page does not point to the branch associated to the current release. Indeed a bit surprising to me, though I am aware that I am not very familiar with the culture here. Nevertheless, there is of course a branch associated to the current release, so that you may wish to consider adding a link to that branch to the official Julia language page.
Switching branches is just basic Git/Github usage.
---
### Author:  [@sgaure](https://discourse.julialang.org/u/sgaure)
#### Post date: [August 17, 2025, 4:56pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/8 "2025-08-17T16:56:44Z")
> [@mraic1](#):
>
> Yes indeed, I have noticed how to get the task reference, but my impression is that references are not very usual in Julia. Am I right?
`Ref`s are used now and then for various purposes, there’s nothing special about them. In this case it’s just a
```julia-auto
t = Ref{Task}()
c = Channel(; taskref=t)
... use t[] ...
```
You’re right that using them for returning values is uncommon in base julia. Typically when things like that are needed in a function, it’s more common that the caller creates the object, which is then mutated by the function.
---
### Author:  [@mraic1](https://discourse.julialang.org/u/mraic1)
#### Post date: [August 17, 2025, 8:49pm UTC](https://discourse.julialang.org/t/exception-control-in-iteration-over-channels-latest-version/131651/9 "2025-08-17T20:49:46Z")
Well … yes, branches are the basic idea of Git, but are people always absolutely aware to check the branch? 🤔 Indeed, I (still) use Git just for browsing, as I am an older and just occasional programmer. Nevertheless, I will try to remember this lesson. Maybe I will agree with you when I get really used to Git. 🙂