Path: utzoo!attcan!uunet!ginosko!xanth!ames!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: optional and key arguments
Message-ID: <28251@news.Think.COM>
Date: 27 Aug 89 21:21:03 GMT
References: <64922@linus.UUCP>
Sender: news@Think.COM
Distribution: comp.lang.lisp
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 61

In article <64922@linus.UUCP> bds@mbunix (Smith) writes:
>(defun test (a b &optional (c 0) &key (d 1))
>(test 5 4 :d 0) results in an error.  
>Is this the Common Lisp standard?

Yes.  See p.61 of CLtL.  Optional arguments are processed before
keyword arguments.

>  If so, does it make sense?  

Yes.  How would you propose that Lisp determine whether the :D is
supposed to be an argument name or the value of the C argument?  In
the above case *you* can tell that it was intended as an argument name
because you gave an incorrect number of arguments, but that isn't always
possible.  Consider:

	(defun test (&optional a b &key d) ...)
	(test :d :e)

This binds A to :D, B to :E, and D to NIL and gets no error.  How
would a Lisp system determine that you actually meant to set the D
argument?

Don't try to special case arguments that are symbols in the KEYWORD
package.  First of all, that would break lots of things; many
functions that keyword-valued arguments.  Second, argument names don't
even have to be in the KEYWORD package; consider:

	(defun test (&optional a b &key ((c-name c) nil)) ...)
	(test 'c-name 'c-val)

This binds A to C-NAME, B to C-VAL, and C to NIL.  Lisp can't tell
that you meant it to be interpreted as

	(test nil nil 'c-name 'c-val)

>In our case, we had 3 optional arguments, and we
>wanted to use the default values, but to set the key argument to
>something else.  

This is why it's generally a bad idea to have both optional and
keyword arguments in the same function.  You should turn them all into
keyword arguments.

>It seems a violation of the notion of optional
>arguments to have to specify them.

Well, you have to specify the Nth optional argument if you want to
specify the N+1th one.  Named arguments can be thought of as a single,
last optional argument which is automatically parsed.  Or they can be
considered as automatically-parsed &REST arguments.  Either of these
analogies will help you remember that optional arguments must be
specified first.

Lisp isn't Ada.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar