Optional arguments hack in Vimscript

optional.vim

function OptLets(name,default)
  let a:let1 = "let a:__OPT_ARGNUM__ = get(a:, '__OPT_ARGNUM__', 0)+1"
  let a:let2 = "let a:".a:name." = get(a:, a:__OPT_ARGNUM__, ".a:default.")"
  
  return a:let1."\n".a:let2
endfunction

command -nargs=+ Optional execute OptLets(<f-args>)

optional_examples.vim

function Fx(a,b,...)
  Optional c 10
  Optional d 20
  
  echo a:a
  echo a:b
  echo a:c
  echo a:d
endfunction

function! TheKnownFunction(arg1,...)
  Optional arg2 0
  Optional arg3 0
  
  echo "and here goes the first argument !"
  if a:arg2
    echo "you supplied the second argument !!"
    if a:arg3
      echo "you went beyond all expectations !!!"
      return "5/5"
    else
      return "4/5"
    endif
  else
    echo "that's it?? :("
    return "2/5"
  endif
endfunction