+1
I've encountered this in a similar context - I have one overall task that
will spawn some number of data fetching tasks, but in some cases I know
that the data fetch call will return no results so I stub the task with
`Task.async(fn -> {:ok, []} end)`. The proposed `Task.completed/1` would be
a great, more semantic replacement and has the benefit of not spawning an
unneeded process.
On Wednesday, June 30, 2021 at 3:22:07 PM UTC-7 Luke Bakken wrote:
> ## Background
>
> I have an enumerable over which I fold and call Task.async based on the
> data in the enumerable. I then Task.yield_many over the list of tasks, and
> use Enum.zip to correlate the original enumerable with the results.
>
> I have a case where, during the fold I find that an entry is invalid for
> running Task.async. It would be convenient to create an "already completed"
> Task that contains an error result. For now, I'm still using Task.async to
> basically return an :error tuple, which of course starts and links a
> process.
>
> Of course, I could work around this by using maps, etc.
>
> I tried using %Task{} to create a "dummy" but calling Task.yield_many with
> such an entry always blocks until the timeout.
>
> ## Proposal
>
> Add Task.completed/1 that creates an "already completed" Task that can
> then be awaited / yielded to return the result used when completed/1 was
> called:
>
> ```elixir
> task = Task.completed({:error, :boom})
> ```
>
> Awaiting or yielding on such a task returns the result immediately without
> invoking a process.
>
> ## Other
>
> .NET has the following to achieve this behavior, for instance:
>
>
> https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.completedtask?view=net-5.0
>
>
> https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.fromresult?view=net-5.0
>
> Thanks for your consideration! If approved I would gladly implement this.
> Luke
>
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