How to pass an optional callback to a proc

[DMisener]

Thank you very much for the excellent explanation. It all makes so much sense 
now.

I combined **PMunch** 's suggustion plus a small variant of your second option. 
to yield:
    
    
    type
      Callback = proc(name: string, length: int)
      
      TestOptions = object
        name: string
        length: int
        callback: Callback
    
    const show = proc(name: string, length: int) =
      echo "Callback executed: ", name, " is ", length, " inches long"
    
    let option1 = TestOptions(name: "foot", length: 12)
    let option2 = TestOptions(name: "inch", length:  1)
    let option3 = TestOptions(name: "yard", length:  36, callback: show)
    
    proc test(options: TestOptions) =
      if options.callback.isNil:
        echo "No callback supplied"
      else:
        options.callback(options.name, options.length)
    
    test option1
    test option2
    test option3
    
    
    Run

which gives the desired result:
    
    
    nim r tests/test_options.nim
    No callback supplied
    No callback supplied
    Callback executed: yard is 36 inches long
    
    
    Run

Lessons learned:

  * A normal **unnamed proc** type defaults to a type **closure** so you can 
have an environment.
  * Procedures are a simply pointer type and so **isNil** is available for 
checking if defined.