Path: utzoo!mnetor!uunet!husc6!rutgers!ames!sdcsvax!ucsdhub!hp-sdd!hplabs!hpcea!hpfcdc!hpfclp!diamant
From: diamant@hpfclp.HP.COM (John Diamant)
Newsgroups: comp.lang.lisp
Subject: Re: Correctness (was Re: Common Lisp lacks portability)
Message-ID: <6950004@hpfclp.HP.COM>
Date: 19 Dec 87 02:31:43 GMT
References: <859@rocky.STANFORD.EDU>
Organization: HP, Fort Collins, CO
Lines: 40

> (defun silly (a b)
>   (declare (integer a b))
>   (loop i from a to b do (print i)))
> 
> Silly does not handle illegal input - its definition promises that
> its input is well-formed.  Safe-silly handles illegal input.

I agree with your example about safe-silly versus silly, but I'd like
to point out that safe-silly needn't be as ugly as you wrote in your
example.  Below your example, see a complete, correct implementation
of safe-silly that doesn't require that bizarre (though quite correct)
let.  Note, that I am not attempting to criticize your understanding
of Common Lisp -- you clearly understand it.  I only wish to point out the
the safe program can be very clean (more so than in your example).
> 
> (defun safe-silly (a b)
>   <Put code here that tests whether the input is legal and takes
>    appropriate action if it isn't.  Note that competent CL programmers
>    know how to return a value from safe-silly without executing the
>    remainder of this function.>
>   (let ((a a) (b b))
>     (declare (integer a b))
>     (loop i from a to b do (print i))))

(defun safe-silly (a b)
	(check-type a integer)
	(check-type b integer)
	(loop i from (the integer a) to (the integer b) do (print i)))

Note that check-type is part of CL and provides an optional string
argument to provide your own error message, and that with the use of the
special form "the," no extra storage is needed.  Clearly, in this example,
the efficiency isn't that significant, but in heavy calculations, it may
well make a big difference.  In a much larger main body, the use of the let
as above may well be worth it (if a and b appear several times).


John Diamant			UUCP:  {hplabs,hpfcla}!hpfclp!diamant
Hewlett Packard Co.		ARPA Internet: diamant%hpfclp@hplabs.HP.COM
Fort Collins, CO