Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!bbn!uwmcsd1!ig!agate!ucbvax!decvax!decwrl!sun!pitstop!sundc!seismo!uunet!cme-durer!klm
From: klm@cme-durer.ARPA (Ken Manheimer)
Newsgroups: comp.emacs
Subject: Re: alist help for an elisp beginner
Summary: The describe-function spiels for fundamental lisp funcs are vacuous
Message-ID: <232@stylus.cme-durer.ARPA>
Date: 18 Feb 88 04:09:19 GMT
References: <276@ho7cad.ATT.COM> <MW4rYsy00WAIyc00P7@andrew.cmu.edu>
Reply-To: klm@cme-amrf.arpa (Ken Manheimer)
Organization: National Bureau of Standards, Gaithersburg, MD
Lines: 68

In article <MW4rYsy00WAIyc00P7@andrew.cmu.edu> jk3k+@ANDREW.CMU.EDU ("Joseph G. Keane") writes:
>Use:
>(defvar file-name-abbreviation-alist
>  (list
>    (cons (format "^%s" (getenv "HOME")) "~")
>    (cons (format "^%s" (getenv "TOOLS")) "<TOOLS>")))
>
>(I bet this looks pretty random if you don't know Lisp.)
>
>--Joe

It's funny; just yesterday i was scrambling around gnuEmacs with
someone who was just learning to maneuver around the editor and its
elisp, and he pointed out a particularly fine little suite of mutually
circular definitions.  Of course, it was the fundamental (and most
beguilingly named) lisp functions that were being "defined".

The quest started with a question about 'assq' and alists; referring
to describe-function, we get:

assq:
Returns non-nil if ELT is the car of an element of LIST.  Comparison
done with eq. The value is actually the element of LIST whose car is ELT.

Well, ok, but if you're a spanking brand new beginner, you're going to
wonder about 'car'.  If you know lisp, you know 'car' means "first
element of a list (or nil for nil)" (or, "Contents of the Accumulator
Register", historically speaking...).

However, if you once again resort to emacs' helping hand,

car:
Return the car of CONSCELL.  If arg is nil, return nil.

Hmm.  CONSCELL.  We've seen 'cons' around, right?  What about that
one?

cons:
Create a new cons, give it CAR and CDR as components, and return it.

Oh, that's much better.  How about cdr?

cdr:
Return the cdr of CONSCELL.  If arg is nil, return nil.

I know that emacs help is not intended to provide a tutorial for or
introduction to lisp.  If you don't have some idea about the list data
structure and its accessors and CONStructors, you ought to start
elsewhere; however i think it's at least a little amusing how
blatantly circular these definitions are.  I mean, the definition for
'cdr' above provides some info about what happens for the boundary
condition nil, but it doesn't say anything about the error that's
provoked when a non-list expression is provided to cdr, and saying
that cdr returns "the cdr of CONSCELL" is *not* an example of a
recursive def, but instead a vacuous one.

Would it hurt, eg for 'cdr', to say something to the effect of "nil
for a null list, the portion of a list after the first element, or
else an error"?  I expect that some such more informative (though less
formal) description might help answer questions like the one Barry
asks, about the difference between 'append' and 'cons'.  (Btw, is this
phrasing sound for dotted pairs?)  Or is the idea specifically to send
the novice scrambling for a real Lisp manual?

Ken Manheimer	 klm@cme-durer.arpa	or	..!uunet!cme-durer.arpa!klm

"For without the inner the outer loses its meaning; and without the
outer, the inner loses its substance."			  R.D.Laing