Task.WaitAll listed in F #

Question

I am doing parallel programming with F #. With a fixed number of elements, for example, with two elements a1, a2 and function f, I can do the following:

let t1 = Task.Factory.StartNew(fun () -> f a1) let t2 = Task.Factory.StartNew(fun () -> f a2) Task.WaitAll(t1, t2) t1.Result, t2.Result 

I wonder how I could do the same with a list of elements:

 let ts = List.map (fun a -> Task.Factory.StartNew(fun () -> fa)) Task.WaitAll(ts) List.map (fun (t: Task<_>) -> t.Result) ts 

Visual Studio notes that Task.WaitAll cannot accept Task <T> as a parameter. Task.WaitAll may have Task [] as an argument, but that doesn't make sense, because I need to get the result for the next calculation.

Answer 1

As Robert explains, if you want to call WaitAll , you will need to cast the sequence of elements to the base type of Task , and then convert them to an array. You can define your extension member for Task to simplify tas:

 type System.Threading.Tasks.Task with static member WaitAll(ts) = Task.WaitAll [| for t in ts -> t :> Task |] 

I use array understanding and instead of Seq.cast instead of Seq.cast , because Seq.cast accepts untyped IEnumerable - so F # indicates the best type for the extension method.

Another option is to not call WaitAll at all - if you do not, the Result properties will be blocked until the task completes. This means that you will block the thread anyway (there might be a bit more locks, but I'm not sure if this affects the performance too much). If you use List.map to collect all the results, the behavior will be almost the same.

Answer 2

This is an unsuccessful project. Task.WaitAll uses the C # params keyword so that you can specify multiple arguments and have them in an array in a method. It also uses C # implicit casting so you can give it a Task<T> . In F # you have to do it yourself, explicitly pronouncing and converting to an array:

 let ts = [| Task.Factory.StartNew(fun () -> 1) Task.Factory.StartNew(fun () -> 2) |] Task.WaitAll(ts |> Seq.cast<Task> |> Array.ofSeq) 

Now you can get the results from ts .

Answer 3

The array also has a map, so there is no reason why you cannot put tasks into the array.

Or you can convert to an array just for waitall ...