How to use with F#?

[sgaliamov]

Hi, I'm trying to use it in F#:

module Program

open System.CommandLine
open System.CommandLine.Invocation;
open System

let [<EntryPoint>] main args =
    let root = new RootCommand()
    let option = new Option<string>("--test")
    root.AddOption option
    root.Handler <- CommandHandler.Create<string>(fun (test: string) -> Console.WriteLine test)
    root.Invoke args

Handler is invoked, but test argument is null. What I'm doing wrong?

[twaalewijn]

I don't think you've done anything wrong in the code example you've posted.

It seems to work just fine for me when copy pasted into a new F# console project targeting netcoreapp3.1 and using version 2.0.0-beta1.20303.1 of System.CommandLine from NuGet.

What doesn't work for me is using the name of another function directly as a handler e.g.:

let handler(test: string) =
    Console.WriteLine test

[<EntryPoint>]
let main args =
    let root = new RootCommand()
    let option = new Option<string>("--test")
    root.AddOption option
    // Compiles just fine but test is null when the handler function gets called.
    root.Handler <- CommandHandler.Create(handler)
    root.Invoke args

I'm assuming this is somehow caused by the implicit cast/conversion from string -> unit to Action<string>.

You can get around this by either passing test as you've done in the example:

let handler(test: string) =
    Console.WriteLine test

[<EntryPoint>]
let main args =
    let root = new RootCommand()
    let option = new Option<string>("--test")
    root.AddOption option
    root.Handler <- CommandHandler.Create(fun (test) -> handler test)
    root.Invoke args

Or defining a type to store your option values in.
Something like this for example:

type Opt(test) =
    member val Test: string = test

let handler(opt: Opt) =
    Console.WriteLine opt.Test

[<EntryPoint>]
let main args =
    let root = new RootCommand()
    let option = new Option<string>("--test")
    root.AddOption option
    root.Handler <- CommandHandler.Create(handler)
    root.Invoke args

[sgaliamov]

@twaalewijn thank you.
i updated the sdk and now it works.