How do you write a SelectMany from a Task in F#

Question

I would like to write a SelectMany monadic bind from a Task in F#. How would I write the following C# code which uses language-ext in F#?

Task<int> result = from task in Task.Run<int>(() => 40) select task + 2;

What are you trying to do? This doesn't compile in C# and would be really weird even if it did. It's not a `SelectMany` either, just a `Select`. — Panagiotis Kanavos, Jul 07 '20 at 11:55
If you have an iterator that produces values asynchronously, you should be using `IAsyncEnumerable` in C#. The equivalent in F# is provided by the [AsyncSeq](https://fsprojects.github.io/FSharp.Control.AsyncSeq/) library — Panagiotis Kanavos, Jul 07 '20 at 11:58
@PanagiotisKanavos you are right! The example C# code I was using was not even correct! — Mike Harris, Jul 07 '20 at 17:57
changed description to say that it works with language-ext for C#. If you the reader, find yourself here and do not understand what a monadic bind is, this is a great example http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html (it uses Haskell but I think is one of the best examples I've seen). — Mike Harris, Jul 07 '20 at 21:25

score 4 · Accepted Answer · answered Jul 07 '20 at 12:56

4

You can use the F# TaskBuilder library to get the F# computation expression (monadic syntax) for tasks. With this, you can rewrite your example as:

let result = task {
  let! t = Task.Run<int>(() => 40)
  return t + 2 }

answered Jul 07 '20 at 12:56

Tomas Petricek

Using base F#, this is what I ended up doing. let task = async { let! x = async { return 40} return x + 2 } – Mike Harris Jul 07 '20 at 17:55
1

Yup! If you do not particularly mind whether you use tasks or asyncs, then this is even better :-) – Tomas Petricek Jul 07 '20 at 19:32

1 Answers1