F# 绑定选项和任务

Question

I want to to replicate the following C# code:

var things = url is null ? null : await GetThings(url);

What I got is:

 task {
            match urlOption with
            | None -> return None
            | Some url ->
                let! things = this.GetThings(url)
                return things |> Some
        }

I excpected to be able to do something like:

let! things = url >>= this.GetThings 

Am I missing some operator declarations? Probably, but do I need to write them on my own or is there a library?

I found Fsharp.Plus but I have no idea whether this does not have what I want, or I just do it wrong.

Answer 1

Yes, you can do it with F#+ but you need to wrap it in an OptionT :

#r "nuget: FSharpPlus"

open System
open System.Threading.Tasks
open FSharpPlus
open FSharpPlus.Data

let x = Task.FromResult (Some 42)
let f x = Task.FromResult (Some x)

let y =
    OptionT x
    >>= (OptionT << f) 
    |> OptionT.run

Then you can use the >>= operator as demonstrated above or the generic `monad`` computation expression.