Subject: Re: Q: optional arguments default values: evaluated when?
From: Erik Naggum <>
Date: 1999/11/08
Newsgroups: comp.lang.lisp
Message-ID: <>

* Matthew Economou <>

  your subject question is precise enough for an answer, so the complex and
  possibly confusing example has been elided.  the default value of an
  optional argument is evaluated in the lexical scope of any previously
  specified arguments, and any free variables are referenced as per the
  usual rules:

((lambda (foo &optional (bar foo) (zot bar)) (list foo bar zot)) t)
==> (t t t)

  or, quoting from the standard: Specifiers for optional parameters

If &optional is present, the optional parameter specifiers are those
following &optional up to the next lambda list keyword or the end of the
list.  If optional parameters are specified, then each one is processed as
follows.  If any unprocessed arguments remain, then the parameter variable
var is bound to the next remaining argument, just as for a required
parameter.  If no arguments remain, however, then init-form is evaluated,
and the parameter variable is bound to the resulting value (or to nil if no
init-form appears in the parameter specifier).  If another variable name
supplied-p-parameter appears in the specifier, it is bound to true if an
argument had been available, and to false if no argument remained (and
therefore init-form had to be evaluated).  Supplied-p-parameter is bound
not to an argument but to a value indicating whether or not an argument had
been supplied for the corresponding var.

  Attention Microsoft Shoppers!  MS Monopoly Money 6.0 are now worthless.