F# Using Return Values From Async

Question

I'm trying to learn F# but I keep running into issues. The latest is using async. In the code below, I'm trying to run two long running operations and perform a calculation based on the result but I get an error "Async does not support the + operator". I have tried casting etc to get it to work but I'm not getting anywhere fast.

Could someone please explain where I'm going wrong.

Thanks.

let SumOfOpFaults =
    async{
        printfn "Getting Sum of Op Faults"
        return query {
            for a in AlarmResult do
            sumBy a.UserFaultTime
        }
    }

let SumOfMcFaults =
    async{
        printfn "Getting Sum of Machine Faults"
        return query {
            for a in AlarmResult do
            sumBy a.MachineFaultTime
        }
    }

[SumOfMcFaults; SumOfOpFaults]
|> Async.Parallel
|> Async.RunSynchronously
|> ignore

let total = SumOfOpFaults + SumOfMcFaults // <---Error Here

Why are you ignoring the result of Async.RunSynchronously? That function will return your two parallel results in an array, then you just add the elements together normally... — TheInnerLight
– TheInnerLight, Commented Sep 13, 2016 at 10:03
Hi - the absolute honest answer is because I don't know what I'm doing, I'm still learning F#. Using your comments, I was able to sort it so thanks - I appreciate the time. If you could post it as an answer, I can accept. — Chico
– Chico, Commented Sep 13, 2016 at 11:35

TheQuickBrownFox · Accepted Answer · 2016-09-13 11:37:47Z

10

SumOfOpFaults is defined as an Async<'T>. It will never change to a 'T so you can't use + on it later.

Async.Parallel turns any sequence of Async computations into one Async computation that runs them in parallel and returns an array.

Async.RunSynchronously doesn't give you a result by side effects, but as a return value. So you just need to do this:

let total =
    [SumOfMcFaults; SumOfOpFaults]
    |> Async.Parallel
    |> Async.RunSynchronously
    |> Array.sum

answered Sep 13, 2016 at 11:37

TheQuickBrownFox

10.7k1 gold badge24 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

F# Using Return Values From Async

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related