I'd like to be able to consume a C# library from F#. Mostly this has been pretty straightforward. However, if I try to call a function that returns a Task<T>
I am not able to get the returned value.
So, I have C# method with the following definition:
public async Task<TEvent> ReadEventAsync<TEvent>(string streamName, int position) where TEvent: class
And I am trying to consume this method from F# as follows:
let readEventFromEventStore<'a when 'a : not struct> (eventStore:IEventStoreRepository) (streamName:string) (position:int) =
async {
return eventStore.ReadEventAsync(streamName, position)
|> Async.AwaitTask
}
I partially apply this function with an instance of an IEventStoreRepository
and the stream name I wish to retrieve the event from:
let readEvent = readEventFromEventStore eventStore streamName
Then, finally, I apply the remaining parameter:
let event = readEvent StreamPosition.Start
When I get the value of event
it is a FSharpAsync<object>
rather than the T
from the Task<T>
that I had expected.
What is the correct method in F# of calling an async
method written in C# with a return type of Task<T>
and accessing the value of T
?
async and awaitInside an async function, you can use the await keyword before a call to a function that returns a promise. This makes the code wait at that point until the promise is settled, at which point the fulfilled value of the promise is treated as a return value, or the rejected value is thrown.
The keyword await passes function control back to the event loop. (It suspends the execution of the surrounding coroutine.) If Python encounters an await f() expression in the scope of g() , this is how await tells the event loop, “Suspend execution of g() until whatever I'm waiting on—the result of f() —is returned.
The await operator is used to wait for a Promise and get its fulfillment value. It can only be used inside an async function or a JavaScript module.
The await operator suspends evaluation of the enclosing async method until the asynchronous operation represented by its operand completes. When the asynchronous operation completes, the await operator returns the result of the operation, if any.
First of all, in your use case, there's no need for the async { }
block. Async.AwaitTask
returns an Async<'T>
, so your async { }
block is just unwrapping the Async
object that you get and immediately re-wrapping it.
Now that we've gotten rid of the unnecessary async
block, let's look at the type you've gotten, and the type you wanted to get. You got an Async<'a>
, and you want an object of type 'a
. Looking through the available Async
functions, the one that has a type signature like Async<'a> -> 'a
is Async.RunSynchronously
. It takes two optional parameters, an int
and a CancellationToken
, but if you leave those out, you've got the function signature you're looking for. And sure enough, once you look at the docs it turns out that Async.RunSynchronously
is the F# equivalent of C#'s sort of (but not exactly) like C#'s await
, which is what you want.await
. C#'s await
is a statement you can use inside an async
function, whereas F#'s Async.RunSynchronously
takes an async
object blocks the current thread until that async
object has finished running. Which is precisely what you're looking for in this case.
let readEventFromEventStore<'a when 'a : not struct> (eventStore:IEventStoreRepository) (streamName:string) (position:int) =
eventStore.ReadEventAsync(streamName, position)
|> Async.AwaitTask
|> Async.RunSynchronously
That should get you what you're looking for. And note that technique of figuring out the function signature of the function you need, then looking for a function with that signature. It'll help a LOT in the future.
Update: Thank you Tarmil for pointing out my mistake in the comments: Async.RunSynchronously
is not equivalent to C#'s await
. It's pretty similar, but there are some important subtleties to be aware of since RunSynchronously
blocks the current thread. (You don't want to call it in your GUI thread.)
Update 2: When you want to await an async result without blocking the current thread, it's usually part of a pattern that goes like this:
The best way to write that pattern is as follows:
let equivalentOfAwait () =
async {
let! result = someAsyncOperation()
doSomethingWith result
}
The above assumes that doSomethingWith
returns unit
, because you're calling it for its side effects. If instead it returns a value, you'd do:
let equivalentOfAwait () =
async {
let! result = someAsyncOperation()
let value = someCalculationWith result
return value
}
Or, of course:
let equivalentOfAwait () =
async {
let! result = someAsyncOperation()
return (someCalculationWith result)
}
That assumes that someCalculationWith
is NOT an async operation. If instead you need to chain together two async operations, where the second one uses the first one's result -- or even three or four async operations in a sequence of some kind -- then it would look like this:
let equivalentOfAwait () =
async {
let! result1 = someAsyncOperation()
let! result2 = nextOperationWith result1
let! result3 = penultimateOperationWith result2
let! finalResult = finalOperationWith result3
return finalResult
}
Except that let!
followed by return
is exactly equivalent to return!
, so that would be better written as:
let equivalentOfAwait () =
async {
let! result1 = someAsyncOperation()
let! result2 = nextOperationWith result1
let! result3 = penultimateOperationWith result2
return! (finalOperationWith result3)
}
All of these functions will produce an Async<'T>
, where 'T
will be the return type of the final function in the async
block. To actually run those async blocks, you'd either do Async.RunSynchronously
as already mentioned, or you could use one of the various Async.Start
functions (Start
, StartImmediate
, StartAsTask
, StartWithContinuations
, and so on). The Async.StartImmediate
example talks a little bit about the Async.SwitchToContext
function as well, which may be something you'll want to read about. But I'm not familiar enough with SynchronizationContext
s to tell you more than that.
An alternative to using the async
computation expression for this situation (F# calling C# Task-based XxxAsync method) is to use the task
computation expression from:
https://github.com/rspeele/TaskBuilder.fs
The Giraffe F# web framework uses task
for more or less the same reason:
https://github.com/giraffe-fsharp/Giraffe/blob/develop/DOCUMENTATION.md#tasks
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