Hi,

Welcome back to the fourth part of Concurrent Programming in fsharp blog post series. In part-2, we just learned that Alt<'a> is a subclass of Job<'a> . In this blog post, we are doing to dive deep into this abstraction and learn what it brings to the table.

An Example

Before diving into the definition of Alt<'a> , let's figure out why we need it in the first place.

Assume that we have a function delayedPrintn which prints a given message after n milliseconds

open Hopac // string -> int -> Job<unit> let delayedPrintn msg delayInMillis = timeOutMillis delayInMillis |> Job.map (fun _ -> printfn "%s" msg)

Executing this function in F# interactive,

#time "on" delayedPrintn "Hi" 3000 |> run #time "off"

will give us the following output

--> Timing now on Hi Real: 00:00:03.000, CPU: 00:00:00.002, GC gen0: 0, gen1: 0 val it : unit = () --> Timing now off

Nothing fancy and it worked as expected.

Let's make it little complicated by defining two more jobs to print Hi and Hello after waiting for 2000 and 1000 milliseconds respectively.

// Job<unit> let delayedHiPrinter = delayedPrintn "Hi" 2000 // Job<unit> let delayedHelloPrinter = delayedPrintn "Hello" 1000

Then define a function to run these two jobs in parallel using the infix operator function <*> from Hopac.Infixes module.

open Hopac.Infixes let runThemParallel () = delayedHiPrinter <*> delayedHelloPrinter |> run |> ignore

If we run this function in F# interactive,

#time "on" runThemParallel () #time "off"

We can witness that the jobs were executed parallelly and print the output as expected.

--> Timing now on Hello Hi Real: 00:00:02.004, CPU: 00:00:00.006, GC gen0: 0, gen1: 0 val it : unit = () --> Timing now off

And here comes the new requirement!

Given we have two printers like the above, if one printer completes its job, stop the other from executing it.

That's interesting! Let's explore how can we solve this

The Alt Type

Alt represents a first-class selective synchronous operation. The idea of alternatives is to allow one to introduce new selective synchronous operations to be used with non-determinic choice. Obviously, when you have a concurrent server that responds to some protocol, you don't have to perform the protocol as a selective synchronous operation. However, if you do encapsulate the protocol as a selective synchronous operation, you can then combine the operation with other selective synchronous operations. That is the essence of Hopac and CML. - Hopac Documentation

The critical point that we are interested in to solve our problem is selective . In other words, among the two printers, we are concerned (selective) in the one which prints first.

The function that can help us here is Alt.choose

Alt.choose creates an alternative that is available when any one of the given alternatives is val choose: seq<#Alt<'x>> -> Alt<'x>

As we are dealing with only two Alt s, we are going to make use of <|> operator function which is an optimised version of calling the choose function with a sequence of two items.

<|> creates an alternative that is available when either of the given alternatives is available. xA1 <|> xA2 is an optimized version of choose [xA1; xA2]. val ( <|> ): Alt<'x> -> Alt<'x> -> Alt<'x> The given alternatives are processed in a left-to-right order with short-cut evaluation. In other words, given an alternative of the form first <|> second, the first alternative is first instantiated and, if it is available, is committed to and the second alternative will not be instantiated at all.

Revisting delayedPrintn function

The delayedPrintn function is returning Job<unit> function now.

val delayedPrintn: string -> int -> Job<unit>

To apply <|> operator function, we need to modify it to return Alt<unit> . The timeOutMillis function is already returning Alt<unit>

val timeOutMillis: int -> Alt<unit>

But the Job.map function transforming it to Job<unit>

val map: ('x -> 'y) -> Job<'x> -> Job<'y>

let delayedPrintn msg delayInMillis = timeOutMillis delayInMillis // Alt<unit> |> Job.map (fun _ -> printfn "%s" msg) // Job<unit>

Alt<'a> is a subclass of Job<'a>

To achieve what we are doing with Job.map , we can make use of the Alt.afterFun function

val afterFun: ('x -> 'y) -> Alt<'x> -> Alt<'y>

- // string -> int -> Job<unit> + // string -> int -> Alt<unit> let delayedPrintn msg delayInMillis = timeOutMillis delayInMillis // Alt<unit> - |> Job.map (fun _ -> printfn "%s" msg) // Job<unit> + |> Alt.afterFun (fun _ -> printfn "%s" msg) // Alt<unit>

Then we can make use of the <|> operator function to choose between the two printers.

// unit -> unit let chooseBetweenThem () = delayedHiPrinter <|> delayedHelloPrinter |> run

If we execute the chooseBetweenThem function with the timer on in F# interactive,

#time "on" chooseBetweenThem () #time "off"

We can verify that it only prints Hello after a seconds delay

--> Timing now on Hello Real: 00:00:01.002, CPU: 00:00:00.004, GC gen0: 0, gen1: 0 val it : unit = () --> Timing now off

Awesome!!

Wait what is happening behind the scene? Was the delayedHiPrinter called?

Yes, It is. But as soon as the delayedHelloPrinter completes its execution, the <|> function stops the execution of delayedHiPrinter and hence we don't see Hi in the output.

To verify this, we can modify the delayedPrintn as below, which prints a log message when printer started its execution

// string -> int -> Alt<unit> let delayedPrintn msg delayInMillis = Alt.prepareFun <| fun _ -> printfn "starting [%s]" msg timeOutMillis delayInMillis |> Alt.afterFun (fun _ -> printfn "%s" msg)

The Alt.prepareFun function that we used here creates an alternative that is computed at instantiation time with the given anonymous function

val prepareFun: (unit -> Alt<'x>) -> Alt<'x>

If we execute the function chooseBetweenThem now, we'll get the following output

--> Timing now on starting [Hi] starting [Hello] Hello Real: 00:00:01.006, CPU: 00:00:00.005, GC gen0: 0, gen1: 0 val it : unit = () --> Timing now off

Negative Acknowledgement

In the above section, we didn't care about the delayedHiPrinter and ignored it completely. But in particular real-world use cases, we can't afford an execution to be stopped abruptly. In those cases, we need to let the Alt<'a> know about this situation.

To implement this kind of scenarios, Hopac offers Negative Acknowledgement.

To implement this behaviour in our example, let's create an another function delayedPrintnWithNack which wraps the delayedPrintn with the negative acknowledgement support.

// string -> int -> Alt<unit> let delayedPrintnWithNack msg delayInMillis = // Alt<'a> -> Alt<unit> let onNack nack = // 1 nack |> Alt.afterFun (fun _ -> printfn "aborting [%s]" msg) Alt.withNackJob <| fun nack -> // 2 Job.start (onNack nack) // 3 |> Job.map (fun _ -> delayedPrintn msg delayInMillis) // 4 // Job<Alt<unit>>

There is a lot is happening in this short code snippet. So, Let's dissect it.

1 We are defining an onNack function to specify what to do in the event of a negative acknowledgement. For simplicity we are just printing an abort message.

2 To make any Alt<'a> negative acknowledgement aware, Hopac provides a function called Alt.withNackJob .

val withNackJob: (Promise<unit> -> Job<Alt<'x>>) -> Alt<'x> The withNackJob function creates an alternative that is computed at instantiation time with the given job constructed with a negative acknowledgement alternative. withNackJob allows client-server protocols that do require the server to be notified when the client aborts the transaction to be encapsulated as selective operations. The negative acknowledgement alternative will be available in case some other instantiated alternative involved in the choice is committed to instead. - Hopac Documentation Promise<'a> is a sub class of Alt<'a> , which we'll see in detail in a later blog post

3 Using the Job.start function, we are immediately starting the onNack job in an another concurrent job

val start: Job<unit> -> Job<unit>

4 After starting the onNack job, we are calling the actual delayedPrintn and return its result.

Let's verify this behaviour with a new set of function.

let delayedHiPrinterWithNack = delayedPrintnWithNack "Hi" 2000 let delayedHelloPrinterWithNack = delayedPrintnWithNack "Hello" 1000 let chooseBetweenThemWithNack () = delayedHiPrinterWithNack <|> delayedHelloPrinterWithNack |> run #time "on" chooseBetweenThemWithNack () #time "off"

--> Timing now on starting [Hi] starting [Hello] Hello aborting [Hi] Real: 00:00:01.000, CPU: 00:00:00.001, GC gen0: 0, gen1: 0 val it : unit = () --> Timing now off

From the log that we can assert that we gracefully handled the negative acknowledgement.

Here is my best effort to show what is happening in the delayedPrintnWithNack function

Summary

In this blog post, we explored how to implement selective synchronisation in Hopac using Alt . It is fascinating to experience that we can write harder concurrent programs with less code.

Stay tuned for the upcoming blog posts. We are going to build some awesome stuff using Alt .

The source code of this blog post is available on GitHub