module Phantom

type Reader = interface end
type Writer = interface end

type Agent<'T> = Agent of string

let makeReader name : Agent<Reader> = Agent name
let makeWriter name : Agent<Writer> = Agent name

let allAgents (a:Agent<_>) = match a with | Agent name -> printfn "Agent %s visiting." name
let onlyWriters (a:Agent<Writer>) = match a with | Agent name -> printfn "Agent %s writing." name

let main () =
    let r = makeReader "reader"
    let w = makeWriter "writer"
    allAgents r
    allAgents w
    onlyWriters r
    onlyWriters w

do main ()