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From: Joshua Taylor <tay...@cs.rpi.edu>
Newsgroups: comp.lang.lisp
Subject: Re: predicate
Date: Wed, 09 Mar 2011 22:25:38 -0500
Organization: A noiseless patient Spider
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On 2011.03.09 9:32 PM, TheFlyingDutchman wrote:
> Is there a way for a macro to handle the &optional parameter
> differently depending on if it is present or not? So that in the case
> of bif, it would generate only an "if then" if the else is not
> supplied, or generate "if then else" when else is supplied?
> 
> ie.
> 
> (bif (= 2 a) (dostuff))
> 
> would not generate code that has a bare "nil" sitting as one branch of
> an if/case/cond?

How about

(defmacro minimal-code-if (test-form
                           then-form
                           &optional
                           (else-form nil else-form-p))
  (if else-form-p
    `(if ,test-form ,then-form ,else-form)
    `(if ,test-form ,then-form)))

or

(defmacro minimal-code-if (test-form
                           then-form
                           &optional
                           (else-form nil else-form-p))
  `(if ,test-form ,then-form
     ,@(when else-form-p
         (list else-form))))

CL-USER> (macroexpand '(minimal-code-if x a b))
(IF X A B)
T

CL-USER> (macroexpand '(minimal-code-if x a))
(IF X A)
T

?