Async Command in fable Elmish - f#

I have this code I'm running using Fable Elmish and Fable remoting to connect to a Suave server. I know that the server works because of postman and there are variations of this code that does call the server
let AuthUser model : Cmd<LogInMsg> =
let callServer = async {
let! result = server.RequestLogIn model.Credentials
return result
}
let result = callServer |> Async.RunSynchronously
match result with
| LogInFailed x -> Cmd.ofMsg (LogInMsg.LogInRejected x)
| UserLoggedIn x -> Cmd.ofMsg (LogInMsg.LogInSuccess x)
The callServer line in the let result fails with Object(...) is not a function, but I don't understand why. Any help would be appreciated.

According to Fable docs Async.RunSynchronously is not supported, though I'm not sure if that is causing your problem. Anyway you should structure your code so that you don't need to block asynchronous computations. In case of Elmish you can use Cmd.ofAsync to create a command out of an async that dispatches messages returned by the async when it completes.
let AuthUser model : Cmd<LogInMsg> =
let ofSuccess result =
match result with
| LogInFailed x -> LogInMsg.LogInRejected x
| UserLoggedIn x -> LogInMsg.LogInSuccess x
let ofError exn = (* Message representing failed HTTP request *)
Cmd.ofAsync server.RequestLogIn model.Credentials ofSuccess ofError
Hopefully this helps.

Related

How to use awaitable inside async?

I'm fairly new to F# and as practice I'm currently writing a module that has a lot of interop with C# libraries.
When library code returns me Task, it's not a problem - I already found a package that enables let! with tasks.
Right now I'm using cliwrap and its ExecuteAsync() method returns not a Task, but some other awaitable type(you can await it in C# and it has GetAwaiter() method).
So far I came up with the following code to use it(please ignore usage of test and (), its temprorary for debugging):
let testLaunchDelayed = async {
let cmd = Cli.Wrap(#"C:\Launcher\Launcher.exe").WithArguments("--TestName " + testName)
let test = cmd.ExecuteAsync().GetAwaiter().GetResult()
()
}
Async.StartImmediate testLaunchDelayed
It seems to work, but using .GetAwaiter().GetResult() feels wrong. Is there a better way ?
Update
Tried using taskbuilder, ended up with this:
async {
let cmd = Cli.Wrap(#"C:\Test\git\tests\func\nunit\AllBin\publish_win10-x64\CommonTestLauncher.exe").WithArguments("--TestName " + testName);
let! cmdTask = task{return! cmd.ExecuteAsync()}
()
}
The issue with using GetResult is that this is a synchronous blocking call. Ideally, you want to use the OnCompleted event to get notified when the operation completes and, only then, access the result. You can encapsulate this into a helper awaitAwaiter, which is much like the Async.AwaitTask operation:
let awaitAwaiter (a:Runtime.CompilerServices.TaskAwaiter<_>) =
Async.FromContinuations(fun (cont, econt, ccont) ->
a.OnCompleted(fun () ->
let res =
try Choice1Of3 (a.GetResult())
with
| :? System.Threading.Tasks.TaskCanceledException as ce -> Choice2Of3 ce
| e -> Choice3Of3 e
match res with
| Choice1Of3 res -> cont res
| Choice2Of3 ce -> ccont (OperationCanceledException("Task has been cancelled", ce))
| Choice3Of3 e -> econt e ) )
This uses FromContinuations to create a new asynchronous computation that uses OnCompleted to get notified when the work is done and then triggers appropriate continuation (handling errors - hopefully correctly, but I have not tested this!)
The use is the same as with AwaitTask:
let testLaunchDelayed testName = async {
let cmd = Cli.Wrap(#"C:\Launcher\Launcher.exe").WithArguments("--TestName " + testName)
let test = cmd.ExecuteAsync().GetAwaiter() |> awaitAwaiter
() }

Is there a way in F# to chain computation?

I would like to create a chain of expressions and any of them can fail when the computation should just stop.
With Unix pipes it is usually like this:
bash-3.2$ echo && { echo 'a ok'; echo; } && { echo 'b ok'; echo; }
a ok
b ok
When something fails the pipeline stops:
echo && { echo 'a ok'; false; } && { echo 'b ok'; echo; }
a ok
I can handle Optionals but my problem is that I might want to do multiple things in each branch:
let someExternalOperation = callToAnAPI()
match someExternalOperation with
| None -> LogAndStop()
| Some x -> LogAndContinue()
Then I would like to keep going with other API calls and only stop if there is an error.
Is there something like that in F#?
Update1:
What I am trying to do is calling out to external APIs. Each call can fail. Would be nice to try to retry but not required.
You can use the F# Async and Result types together to represent the results of each API Call. You can then use the bind functions for those types to build a workflow in which you only continue processing when the previous calls were successful. In order to make that easier, you can wrap the Async<Result<_,_>> you would be working with for each api call in its own type and build a module around binding those results to orchestrate a chained computation. Here's a quick example of what that would look like:
First, we would lay out the type ApiCallResult to wrap Async and Result, and we would define ApiCallError to represent HTTP error responses or exceptions:
open System
open System.Net
open System.Net.Http
type ApiCallError =
| HttpError of (int * string)
| UnexpectedError of exn
type ApiCallResult<'a> = Async<Result<'a, ApiCallError>>
Next, we would create a module to work with ApiCallResult instances, allowing us to do things like bind, map, and return so that we can process the results of a computation and feed them into the next one.
module ApiCall =
let ``return`` x : ApiCallResult<_> =
async { return Ok x }
let private zero () : ApiCallResult<_> =
``return`` []
let bind<'a, 'b> (f: 'a -> ApiCallResult<'b>) (x: ApiCallResult<'a>) : ApiCallResult<'b> =
async {
let! result = x
match result with
| Ok value ->
return! f value
| Error error ->
return Error error
}
let map f x = x |> bind (f >> ``return``)
let combine<'a> (acc: ApiCallResult<'a list>) (cur: ApiCallResult<'a>) =
acc |> bind (fun values -> cur |> map (fun value -> value :: values))
let join results =
results |> Seq.fold (combine) (zero ())
Then, you would have a module to simply do your API calls, however that works in your real scenario. Here's one that just handles GETs with query parameters, but you could make this more sophisticated:
module Api =
let call (baseUrl: Uri) (queryString: string) : ApiCallResult<string> =
async {
try
use client = new HttpClient()
let url =
let builder = UriBuilder(baseUrl)
builder.Query <- queryString
builder.Uri
printfn "Calling API: %O" url
let! response = client.GetAsync(url) |> Async.AwaitTask
let! content = response.Content.ReadAsStringAsync() |> Async.AwaitTask
if response.IsSuccessStatusCode then
let! content = response.Content.ReadAsStringAsync() |> Async.AwaitTask
return Ok content
else
return Error <| HttpError (response.StatusCode |> int, content)
with ex ->
return Error <| UnexpectedError ex
}
let getQueryParam name value =
value |> WebUtility.UrlEncode |> sprintf "%s=%s" name
Finally, you would have your actual business workflow logic, where you call multiple APIs and feed the results of one into another. In the below example, anywhere you see callMathApi, it is making a call to an external REST API that may fail, and by using the ApiCall module to bind the results of the API call, it only proceeds to the next API call if the previous call was successful. You can declare an operator like >>= to eliminate some of the noise in the code when binding computations together:
module MathWorkflow =
let private (>>=) x f = ApiCall.bind f x
let private apiUrl = Uri "http://api.mathjs.org/v4/" // REST API for mathematical expressions
let private callMathApi expression =
expression |> Api.getQueryParam "expr" |> Api.call apiUrl
let average values =
values
|> List.map (sprintf "%d")
|> String.concat "+"
|> callMathApi
>>= fun sum ->
sprintf "%s/%d" sum values.Length
|> callMathApi
let averageOfSquares values =
values
|> List.map (fun value -> sprintf "%d*%d" value value)
|> List.map callMathApi
|> ApiCall.join
|> ApiCall.map (List.map int)
>>= average
This example uses the Mathjs.org API to compute the average of a list of integers (making one API call to compute the sum, then another to divide by the number of elements), and also allows you to compute the average of the squares of a list of values, by calling the API asynchronously for each element in the list to square it, then joining the results together and computing the average. You can use these functions as follows (I added a printfn to the actual API call so it logs the HTTP requests):
Calling average:
MathWorkflow.average [1;2;3;4;5] |> Async.RunSynchronously
Outputs:
Calling API: http://api.mathjs.org/v4/?expr=1%2B2%2B3%2B4%2B5
Calling API: http://api.mathjs.org/v4/?expr=15%2F5
[<Struct>]
val it : Result<string,ApiCallError> = Ok "3"
Calling averageOfSquares:
MathWorkflow.averageOfSquares [2;4;6;8;10] |> Async.RunSynchronously
Outputs:
Calling API: http://api.mathjs.org/v4/?expr=2*2
Calling API: http://api.mathjs.org/v4/?expr=4*4
Calling API: http://api.mathjs.org/v4/?expr=6*6
Calling API: http://api.mathjs.org/v4/?expr=8*8
Calling API: http://api.mathjs.org/v4/?expr=10*10
Calling API: http://api.mathjs.org/v4/?expr=100%2B64%2B36%2B16%2B4
Calling API: http://api.mathjs.org/v4/?expr=220%2F5
[<Struct>]
val it : Result<string,ApiCallError> = Ok "44"
Ultimately, you may want to implement a custom Computation Builder to allow you to use a computation expression with the let! syntax, instead of explicitly writing the calls to ApiCall.bind everywhere. This is fairly simple, since you already do all the real work in the ApiCall module, and you just need to make a class with the appropriate Bind/Return members:
type ApiCallBuilder () =
member __.Bind (x, f) = ApiCall.bind f x
member __.Return x = ApiCall.``return`` x
member __.ReturnFrom x = x
member __.Zero () = ApiCall.``return`` ()
let apiCall = ApiCallBuilder()
With the ApiCallBuilder, you could rewrite the functions in the MathWorkflow module like this, making them a little easier to read and compose:
let average values =
apiCall {
let! sum =
values
|> List.map (sprintf "%d")
|> String.concat "+"
|> callMathApi
return!
sprintf "%s/%d" sum values.Length
|> callMathApi
}
let averageOfSquares values =
apiCall {
let! squares =
values
|> List.map (fun value -> sprintf "%d*%d" value value)
|> List.map callMathApi
|> ApiCall.join
return! squares |> List.map int |> average
}
These work as you described in the question, where each API call is made independently and the results feed into the next call, but if one call fails the computation is stopped and the error is returned. For example, if you change the URL used in the example calls here to the v3 API ("http://api.mathjs.org/v3/") without changing anything else, you get the following:
Calling API: http://api.mathjs.org/v3/?expr=2*2
[<Struct>]
val it : Result<string,ApiCallError> =
Error
(HttpError
(404,
"<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Error</title>
</head>
<body>
<pre>Cannot GET /v3/</pre>
</body>
</html>
"))

F# Async.Map not found

I have source from http://sourceforge.net/p/onvifdm/code/HEAD/tree/
When I try to build this solution, I get the following issue in the onvif.utils project.
use! response = request.AsyncGetResponse() |> Async.Map(fun x-> x:?>HttpWebResponse)
Error: Method or object constructor 'Map' not found
D:\Working\onvifdm\onvifdm-code-96-trunk\onvif\onvif.utils\OdmSession.fs
Here's a function that extends Async, taken from a useful source on understanding map.
module Async =
let map f op = async {
let! x = op
let value = f x
return value
}
Which has a type signature ('a -> 'b) -> Async<'a> -> Async<'b>
For example -
let onInts = async { return 1 }
... is an async<int>, and ...
let onStrings = Async.map (fun x -> x.ToString()) onInts
... produces an Async<string>.
You can use following code as a replacement:
let! webResponse = request.AsyncGetResponse()
use response = webResponse:?>HttpWebResponse
Also you can write your own extension to Async type - here is an example how it can be done. All you need is to implement high order function for the code above.
For those still looking for an answer to this, the problem is that Async is referencing the wrong class. Change it to Microsoft.FSharp.Control.Async and the problem should be resolved.
if this is still an open question, let me suggest using FsToolkit.ErrorHandling:
[https://www.nuget.org/packages/FsToolkit.ErrorHandling]
It contains an Async.map implementation and a lot of useful features to deal with Result.
As an alternative I think this should work:
use! response = request.AsyncGetResponse() |> async.Bind (fun x-> x:?>HttpWebResponse |> async.Return)

FSharp: Using CSV Type Provider Async

I am using the csv type provider to collect some data from a series of files I have on Azure blob storage:
#r "../packages/FSharp.Data.2.0.9/lib/portable-net40+sl5+wp8+win8/FSharp.Data.dll"
open FSharp.Data
type censusDataContext = CsvProvider<"https://portalvhdspgzl51prtcpfj.blob.core.windows.net/censuschicken/AK.TXT">
type stateCodeContext = CsvProvider<"https://portalvhdspgzl51prtcpfj.blob.core.windows.net/censuschicken/states.csv">
let stateCodes = stateCodeContext.Load("https://portalvhdspgzl51prtcpfj.blob.core.windows.net/censuschicken/states.csv");
let fetchStateData (stateCode:string)=
let uri = System.String.Format("https://portalvhdspgzl51prtcpfj.blob.core.windows.net/censuschicken/{0}.TXT",stateCode)
censusDataContext.Load(uri).Rows
let usaData = stateCodes.Rows
|> Seq.collect(fun r -> fetchStateData(r.Abbreviation))
|> Seq.length
I now want to run these async and I am running into a problem with AsyncLoad:
let fetchStateDataAsync(stateCode:string)=
async{
let uri = System.String.Format("https://portalvhdspgzl51prtcpfj.blob.core.windows.net/censuschicken/{0}.TXT",stateCode)
let! stateData = censusDataContext.AsyncLoad(uri)
return stateData.Rows
}
let usaData = stateCodes.Rows
|> Seq.collect(fun r -> fetchStateDataAsync(r.Abbreviation))
|> Seq.length
The error message is
The type 'Async<seq<CsvProvider<...>.Row>>' is not compatible with the type 'seq<'a>'
Forgive my lack of async knowledge, but do I have to use something other than Seq.Collect when applying async functions?
Thanks in advance
The problem is that turning code to asynchronous (by wrapping it in the async { .. } block) changes the result from seq<Row> to Async<seq<Row>> - that is, you now get an asynchronous computation that will eventually complete and return the sequence.
To fix this, you need to somehow start the computation and wait for the result. There is a number of choices - like running one by one sequentially. Probably the easiest option (and maybe the best - depending on what you want to do) is to run the computations in parallel:
let getAll =
stateCodes.Rows
|> Seq.map(fun r -> fetchStateDataAsync(r.Abbreviation))
|> Async.Parallel
This gives you an asynchronous computation that runs all the downloads and returns an array of results. You can run this synchronously (and block) and get the results:
getAll |> Async.RunSynchronously
|> Seq.collect id
|> Seq.length
If you want to run the downloads asynchronously in the background you can do that to, but you need to specify what to do with the result. For example:
async {
let! all = getAll
all |> Seq.collect id |> Seq.length |> printfn "Length %d" }
|> Async.Start

Why is my MailboxProcessor hanging?

I can't work out why the following code is hanging at the call to GetTotal. I don't seem to be able to debug inside the MailboxProcessor, so it's hard to see what's going on.
module Aggregator
open System
type Message<'T, 'TState> =
| Aggregate of 'T
| GetTotal of AsyncReplyChannel<'TState>
type Aggregator<'T, 'TState>(initialState, f) =
let myAgent = new MailboxProcessor<Message<'T, 'TState>>(fun inbox ->
let rec loop agg =
async {
let! message = inbox.Receive()
match message with
| Aggregate x -> return! loop (f agg x)
| GetTotal replyChannel ->
replyChannel.Reply(agg)
return! loop agg
}
loop initialState
)
member m.Aggregate x = myAgent.Post(Aggregate(x))
member m.GetTotal = myAgent.PostAndReply(fun replyChannel -> GetTotal(replyChannel))
let myAggregator = new Aggregator<int, int>(0, (+))
myAggregator.Aggregate(3)
myAggregator.Aggregate(4)
myAggregator.Aggregate(5)
let totalSoFar = myAggregator.GetTotal
printfn "%d" totalSoFar
Console.ReadLine() |> ignore
It seems to work fine when using an identical MailboxProcessor directly, rather than wrapping in the Aggregator class.
The problem is that you did not start the agent. You can either call Start after you create the agent:
let myAgent = (...)
do myAgent.Start()
Alternatively, you can create the agent using MailboxProcessor<'T>.Start instead of calling the constructor (I usually prefer this option, because it looks more functional):
let myAgent = MailboxProcessor<Message<'T, 'TState>>.Start(fun inbox -> (...) )
I suppose that you couldn't debug the agent, because the code inside agent wasn't actually running. I tried adding printfn "Msg: %A" message right after the call to Receive inside the agent (to print incoming messages for debugging) and I noticed that, after calling Aggregate, no messages were actually received by the agent... (It only blocked after calling GetTotal, which avaits reply)
As a side-note, I would probably turn GetTotal into a method, so you'd call GetTotal(). Properties are re-evaluated each time you access them, so your code does the same thing, but best practices don't recommend using properties that do complex work.
You forgot to start the mailbox:
open System
type Message<'T, 'TState> =
| Aggregate of 'T
| GetTotal of AsyncReplyChannel<'TState>
type Aggregator<'T, 'TState>(initialState, f) =
let myAgent = new MailboxProcessor<Message<'T, 'TState>>(fun inbox ->
let rec loop agg =
async {
let! message = inbox.Receive()
match message with
| Aggregate x -> return! loop (f agg x)
| GetTotal replyChannel ->
replyChannel.Reply(agg)
return! loop agg
}
loop initialState
)
member m.Aggregate x = myAgent.Post(Aggregate(x))
member m.GetTotal = myAgent.PostAndReply(fun replyChannel -> GetTotal(replyChannel))
member m.Start() = myAgent.Start()
let myAggregator = new Aggregator<int, int>(0, (+))
myAggregator.Start()
myAggregator.Aggregate(3)
myAggregator.Aggregate(4)
myAggregator.Aggregate(5)
let totalSoFar = myAggregator.GetTotal
printfn "%d" totalSoFar
Console.ReadLine() |> ignore

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