Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!ukma!mailrus!ames!ai.etl.army.mil!hoey
From: hoey@ai.etl.army.mil (Dan Hoey)
Newsgroups: comp.lang.lisp
Subject: Re: Summing a list
Message-ID: <200@ai.etl.army.mil>
Date: 31 Oct 88 17:04:44 GMT
References: <10794@srcsip.UUCP> <10813@srcsip.UUCP> <249@pitstop.UUCP> <3780@cs.utexas.edu> <29856@think.UUCP>
Reply-To: hoey@ai.etl.army.mil (Dan Hoey)
Organization: Naval Research Lab, Washington, DC
Lines: 22

In article <29856@think.UUCP> barmar@kulla.think.com.UUCP (Barry Margolin)
	writes:
...
>I don't know why :KEY was left out.  In my opinion, the best way to do
>this currently is

>	(flet ((+-weight (x y)
>		 (+ (box-weight x) (box-weight y))))
>	  (reduce #'+-weight ...))

The problem there is that if you do that with more than two elements, you end
up trying to take the box-weight of the sum of two box-weights, and numbers
aren't boxes.  The ``right'' way is

	(flet ((+-weight (x y)
		 (+ x (box-weight y))))
	  (reduce #'+-weight ... :initial-value 0))

Guy has explained the omission of :KEY based on some feature of REDUCE, but I
don't remember it being extremely convincing.

Dan