How to pass an optional callback to a proc

[DMisener]

Given the following examplar:
    
    
    import std/options
    
    type
      Callback = Option[proc(name: string, length: int)]
      
      TestOptions = object
        name: string
        length: int
        callback: Callback
    
    proc show(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: some(show)) 
# FIXME: uncomment
    
    proc test(options: TestOptions) =
      echo "Options = ", options
      
      if options.callback.isSome:
        options.callback.get()(options.name, options.length)
      else:
        echo "No callback supplied"
    
    test  option1
    test  option2
    #test option3 # FIXME: uncomment
    
    
    Run

Running the code gives:
    
    
    nim r tests/test_options.nim
    Options = (name: "foot", length: 12, callback: ...)
    No callback supplied
    Options = (name: "inch", length: 1, callback: ...)
    No callback supplied
    
    
    Run

If I uncomment the two **FIXME** lines to test actually supplying a callback, I 
get the following compile error:
    
    
    nim r tests/test_options.nim
    /Users/DM/work/trunk/nim/tests/test_options.nim(16, 62)
    Error: type mismatch: got 'Option[proc (name: string, length: 
int){.gcsafe.}]' for 'some(show)' but
     expected 'Callback = Option[proc (name: string, length: int){.closure.}]'
    
    
    Run

Anyone know the _magic incantation_ to allow the code to compile and hopefully 
run? I tried sprinkling {.closure.} and {.gcsafe.} pragmas around with no 
success.