Async F# Question

Program.fs

namespace fsharp

open System
open System.Collections.Generic
open System.IO
open System.Linq
open System.Threading.Tasks
open Microsoft.AspNetCore
open Microsoft.AspNetCore.Hosting
open Microsoft.Extensions.Configuration
open Microsoft.Extensions.Hosting
open Microsoft.Extensions.Logging
open ServiceStack // <----- try comment this and `async {` will not be errored by compiller

module AsyncDemo =
    type Student = {
        name: string
        age: int
        subjects: string []  
    }

    type IStudentApi =
        { allStudents : Async<Student list>
          findByName : string -> Async<Student option> }

    let students() = [
        { name = "Alice"; age = 20; subjects = [| "math" |]  }
        { name = "Martin"; age = 19; subjects = [| "cs" |]  }
    ]

    let studentApi : IStudentApi = {
        allStudents = async { return students() }
        findByName = fun name -> async {
            let foundStudent = 
                students()
                |> List.tryFind (fun student -> student.name = name)

            return foundStudent  
        }
    }

module Program =
    let exitCode = 0

    let CreateHostBuilder args =
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(fun webBuilder ->
                ModularExtensions.UseModularStartup<Startup>(webBuilder) |> ignore
            )

    [<EntryPoint>]
    let main args =
        CreateHostBuilder(args).Build().Run()

        exitCode