How to execute two Async<> calls with different result types in parallel?

Question

I have two asynchronous operations, so I have these functions:

// Returs Async<Person>
let GetPerson =
  ...

// Returs Async<Address>
let GetAddress =
  ...

What is the idiomatic way to execute them in parallel and get their results?

My starting point is this approach.

let MyFunc = async {
    let! person = GetPerson()
    let! address = GetAddress()
    ...
  }

This works, but this runs the two operations sequentially.

I also tried this (sort of based on my C# experience).

let MyFunc = async {
    let personA = GetPerson()
    let addressA = GetAddress()
    let! person = personA
    let! address = addressA
    ...
  }

But it doesn't work, it also runs the two operations sequentially.

What most of the documentation says is to use Async.Parallel with a sequence, but the problem is that the result type of the two operations are different, so I cannot put them in a sequence.

let MyFunc = async {
    let personA = GetPerson()
    let addressA = GetAddress()

    [personA; addressA]
    |> Async.Parallel
    ...
  }

This gives a compilation error, because the two values have different types. (And also, with this syntax, how could I get the actual results?)

What is the idiomatic way to do this?

Tomas Petricek · Accepted Answer · 2016-12-29 20:25:51Z

10

The idiomatic approach is to start both computations using Async.StartAsChild and then wait for their completion using a second let!:

let MyFunc = async {
    let! personWork = GetPerson() |> Async.StartChild
    let! addressWork = GetAddress() |> Async.StartChild
    let! person = personWork
    let! address = addressWork
    // (...)
  }

Just calling GetPerson does not actually start the work - unlike in C# where tasks are created started, F# workflows are just descriptions of the work to be done, so they need to be started explicitly. The Async.StartChild operation gives you a workflow that starts the work and returns another workflow that can be used for wait for its completion.

answered Dec 29, 2016 at 20:25

Tomas Petricek

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6 Comments

Chinwobble Over a year ago

IF GetPerson and GetAddress are also async primitives, they by convention should they also be prefixed with async. i.e. asyncGetPerson and asyncGetAddress ?

Tomas Petricek Over a year ago

That is certainly the case for F# library functions (AsyncGetResponse vs. GetResponse). I'd follow this pattern if I had both synchronous and asynchronous versions, but probably not when writing code that only has asynchronous version.

Gus Over a year ago

@TomasPetricek I noticed that for Async.Merge on your joinads blog you used Async.Parallel instead of this 'idiomatic' approach. I'm curious about why did you use such approach, is it more efficient for a specific scenario?

knocte Over a year ago

@Gustavo: Async.Parallel only works for async jobs that return the same type, but this S.O. question is about differing types

Gus Over a year ago

@knocte I know, but if you have a look at the code I was referring you'll see it uses boxing to allow the types to differ.

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