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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Partial evaluator sought
Message-ID: <1991May10.055107.3185@Think.COM>
Date: 10 May 91 05:51:07 GMT
References: <13521@pasteur.Berkeley.EDU>
Sender: news@Think.COM
Reply-To: barmar@think.com
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 32

In article <13521@pasteur.Berkeley.EDU> boyland@sequoia.Berkeley.EDU (John B. Boyland) writes:
>I'm looking for a general purpose Common Lisp partial evaluator;
>a function which evaluates pure Lisp code if all values are known:

The following should work in a Common Lisp that includes the condition
system:

(defun partial-eval (form)
  (handler-case ()
      (values (eval form) t)
    ((or unbound-variable undefined-function)
     (values nil nil))))

Actually, I would suggest a macro form, so that the form would be evaluated
in the correct lexical context:

(defmacro partial-eval (&body body)
  `(handler-case ()
       (progn .,body)
     ((or unbound-variable undefined-function)
      (values nil nil))))

This is similar to the definition of WITHOUT-ERRORS, except that it only
catches certain errors.  Another difference in this version is that it
returns all the values of the body in the non-error case.  If the
application needs to distinguish the error from non-error case, then it
must make sure that NIL, NIL is not a valid set of values of the body.
-- 
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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