Re: Optional and Default Parameters

[Jim Meehan <Meehan@YALE.ARPA>]

    Are there  any  plans  to include something on the order of Zetalisp
    &Optional field in the parameter list for functions?  ...

It's not hard to build that on top of T, although if you add  the  whole
bunch of  &-keywords,  it gets very messy.  I use the following notation
for specifying optional parameters that don't all have to  come  at  the
end:

(DEFINE-OPT (name . specs) (defaults) . code)

where each  spec  is  either  a symbol, signifying a required parameter,
or  a  list  containing  optional  parameters.   The  depth  of  nesting
indicates the order of preference; obviously, no two optional parameters
should be nested at the same depth.  Example:

(DEFINE-OPT (SET-DRAW-COLOR (BITMAP (COLOR-MAP)) NEW-COLOR)
            ((CURRENT-BITMAP)
             (CURRENT-COLOR-MAP))
  (SET (ELT COLOR-MAP (DRAW-COLOR-INDEX BITMAP)) NEW-COLOR))

This would be documented as

    (SET-DRAW-COLOR {bitmap {color-map}} color) -> color

    This  routine  sets  the  color  used  for  drawing lines on bitmap.
    Bitmap  defaults  to  (CURRENT-BITMAP)  and  color-map  defaults  to
    (CURRENT-COLOR-MAP).

In other  words,  if  there is only one parameter, it must be the color.
If there are two, they're assumed  to  be  the  bitmap  and  the  color,
because the  bitmap  parameter  is not as deeply nested as the color-map
parameter.  The defaults are given in  left-to-right  order,  regardless
of the  nesting  depth  of  the  optional  parameters for which they are
the defaults.  Defaults are closed  outside  the  procedure  definition.

The code to do this  is  pretty  simple.   (See  <A.PEARL.MEEHAN>LEXPR.T
on the Research 20.)  Lots of variations are possible.
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