A subtle(?) way to break option parsing in Pony (option vs. arg)

broken.diff

--- main.pony	Sun Apr  7 09:36:46 2019
+++ main.pony.broken	Sun Apr  7 09:37:31 2019
@@ -25,5 +25,5 @@
           env.exitcode(1)
           return
       end
-    let port: String = cmd.option("port").string()
+    let port: String = cmd.arg("port").string()
     env.out.print("Port="+port)

works.pony

use "cli"

actor Main
  new create(env: Env) =>
    let cs =
      try
        CommandSpec.leaf("echo", "A sample echo program", [
          OptionSpec.string("port", "TCP port to listen on."
            where short' = 'P', default' = "70")
        ])? .> add_help()?
      else
        env.exitcode(-1)  // some kind of coding error
        return
      end

    let cmd =
      match CommandParser(cs).parse(env.args, env.vars)
      | let c: Command => c
      | let ch: CommandHelp =>
          ch.print_help(env.out)
          env.exitcode(0)
          return
      | let se: SyntaxError =>
          env.out.print(se.string())
          env.exitcode(1)
          return
      end
    let port: String = cmd.option("port").string()
    env.out.print("Port="+port)