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QUESTION
I'm very new to F# and I've been reading F# for Fun and Profit. In the Why use F#? series, there's a post describing async code. I ran across the Async.StartChild
function and I don't understand why the return value is what it is.
The example:
...ANSWER
Answered 2021-May-29 at 05:37The idea is this: You start another async-computation (in the background) with let wait = Async.StartChild otherComp
and you get the waiter back.
What that means is that let! result = waiter
will block and wait for the result of your background-computation whenever you want.
if Async.StartChild
would return a Async<'t>
you would wait with let! x = otherComp
right there and it would be just like a normal let! result = otherComp`
And yes F# Async-Workflows will only start once you do something like Async.Start...
or Async.RunSynchronously
(it's not like a Task
that usually runs as soon as you created it)
that's why in C# you can create a Task (var task = CreateMyTask()
) at one point (that would be the Async.StartChild
part) and then later use var result = await task
to wait there for the result (that's the let! result = waiter
part).
Async.StartChild
returns Async<'T>>
instead of Async<'T>
This is because the workflows started this way is supposed to behave like a child-task/process. When you cancel the containing workflow the child should be canceled as well.
So on a technical level the child-workflow needs access to the cancelation-token without you passing it explicitly and that is one thing the Async
-Type handles in the background for you when you use Bind
(aka let!
here).
So it has to be this type in order for that passing of the cancelation-token to work.
QUESTION
I can’t find a simple working example of the use of Map.fold
anywhere. I have seen the F# for fun and profit and the MSFT documents on ‘fold’.
Can you provide a short working example of the use of Map.fold
?
ANSWER
Answered 2021-Mar-13 at 23:13let capitals =
[("Australia", "Canberra"); ("Canada", "Ottawa"); ("China", "Beijing");
("Denmark", "Copenhagen"); ("Egypt", "Cairo"); ("Finland", "Helsinki");
("France", "Paris"); ("Germany", "Berlin"); ("India", "New Delhi");
("Japan", "Tokyo"); ("Mexico", "Mexico City"); ("Russia", "Moscow");
("Slovenia", "Ljubljana"); ("Spain", "Madrid"); ("Sweden", "Stockholm");
("Taiwan", "Taipei"); ("USA", "Washington D.C.")]
|> Map.ofList
printfn "%A\n" capitals
printfn "%s"
(Map.fold
(fun acc key value -> (acc + "(" + key + "," + value + ")"))
""
capitals)
QUESTION
I looked up the documentation of printfn here and this is what it says:
printfn format
Print to stdout using the given format, and add a newline.
format : TextWriterFormat<'T> The formatter.
Returns: 'T The formatted result.
But if I type the following in FSI
...ANSWER
Answered 2021-Feb-05 at 14:24It's not inconsistent. In your example, the format is of type TextWriterFormat
, so the final return type is unit
, because 'T
is unit
.
If you had written let v = printfn "Hello %s"
, then the type of v
would be string -> unit
. This is how F# provides type-safe string formatting.
QUESTION
Async.SwitchSynchronizationContext allows an Async
action to switch to running within a given SynchronizationContext
. I would like to synchronously begin an Async
computation within a SynchronizationContext
, rather than switching inside the Async
.
This would ensure that Async
actions are run in the desired order, and that they are not run concurrently.
Is this possible? The documentation for Async.SwitchSynchronizationContext
mentions using SynchronizationContext.Post
, but no such functionality is exposed for Async
.
ANSWER
Answered 2021-Jan-26 at 11:43I have not tested this, but I think the easiest way to achieve what you want is to combine the SynchronizationContext.Send
method (mentioned in the comments) with the Async.StartImmediate
operation. The former lets you start some work synchronously in the synchronization context. The latter lets you start an async workflow in the current context.
If you combine the two, you can define a helper that starts a function in a given synchronization context and, in this function, immediately starts an async workflow:
QUESTION
I'm trying to invoke the Map.change
function. It's clearly defined in the documentation but it just doesn't work for me. I've tried opening different namespaces (such as FSharp.Core
, Microsoft.FSharp.Collections
), with no success.
ANSWER
Answered 2020-Nov-05 at 14:27Your code works (apart from the printf
). Maybe check your F# or .NET (Core) version. The Collections namespace doesn't have to be open.
QUESTION
I have a JHipster project with two microservices and a gateway. Both were generated through JHipster. I am trying to send an object from Projects
Microservice with Spring Cloud Stream with Kafka
to ChargeCodes microservice. For this, I added Spring Cloud Stream code in ProjectResource.java file. Based on my requirement, Whenever new project object created, Spring Cloud Stream should send it to Kafka topic and Kafka consumer in the second microservice consumes it. To make it simple ,I created producer and consumer in the same project. When I compile the microservice I get following exception.
ANSWER
Answered 2018-Feb-06 at 19:43From a first glance it appears that something caused one or more rows in DATABASECHANGELOG to be deleted, so Liquibase things that the changeset to create the table projects.address gets run again, but the table already exists.
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