How to use FSharpx TaskBuilder with functions taki

问题:

I have been lately programming with the FSharpx library and especially its TaskBuilder. Now I wonder if it should be possible to define a function which takes parameters and takes a result. Such as

 let doTask(parameter:int) =
     let task = TaskBuilder(scheduler = TaskScheduler.Current)        
     task {
         return! Task.Factory.StartNew(fun() -> parameter + 1)
     } 

 match FSharpx.Task.run doTask(1) with
 | _ -> ()

Looking at the source code I see run expects a function taking no parameters and returning a Task<'a>. There doesn't look like being examples on FSharpx TaskTests either.

I'd appreciate if someone could advice how should I get a scenario like this going with FSharpx or if one isn't supposed to use the library like this for a reason I haven't quite grasped as of yet.

<edit: I believe I could wrap doTask as follows

 wrapperDoTask() = doTask(101)
 match FSharpx.Task.run wrapperDoTask with
 | _ -> ()

And it might work. I'm not with a compiler currently, so this is a bit of a handwaving. Does anyone have an opinion on any direction or did I just answer my own question? :)

<edit2: I think I need to edit this one more time based on MisterMetaphor's answer. Especially his P.S., I think, was well informing. I use FSharpx TaskBuilder to interop with C#, in which, as noted, tasks are returned as hot (with some minor exceptions), already running. This is in connection with my recent question Translating async-await C# code to F# with respect to the scheduler and in relation Orleans (I'll add some tags to beef up the context, maybe someone else is pondering these too).

When thinking in C# terms, what I try to achieve is to await the task result before returning, but without blocking. The behaviour I'm after is especially of that of await not .Result. The difference can be read, for instance, from

• Await, and UI, and deadlocks! Oh my!
• Don't Block on Async Code.

Trying to think which context or scheduler or behavior or something is going on in terms of C# is somewhat fuzzy for me. Unfortunatelly it looks like I can't ignore all the details when it comes to interop. :)

回答1:

You need to use Task.run only if you want to wait for the task completion synchronously on the current thread. It takes a single parameter and you can consider that parameter a task factory -- i.e. a means to create a Task<_>. Unlike Async<_>, the Task<_> starts running as soon as it is created. That is not always a desirable behavior.

You could achieve similar results (a blocking wait for task completion) with (doTask 101).Result, but I think Task.run is more idiomatic to F#, in a way that it uses a Result return type to signal an error instead of raising an exception. It might be arguable which is better, depending on situation, but in my experience in simpler cases a special result type is more composable than exceptions.

Another point here is that you should avoid blocking waits (Task.run, .Wait(), .Result) as much as you can. (Ideally, you'd have one of those only at the top level of your program.)

P.S. This if out of scope of the question, but your doTask function looks funny. task { return! Task.Factory.StartNew( ... ) } is equivalent to Task.Factory.StartNew( ... ). What you probably wanted to do is task { return parameter + 1 }.

EDIT

So, in response to OP's question edit :) If you need the await behavior from C#, you just need to use let! .... Like this:

task {
    let! x = someTask 1 2 3
    return x + 5
}