From ...
From: Erik Naggum <clerik@naggum.no>
Subject: Re: progv question
Date: 1997/10/19
Message-ID: <3086248700500260@naggum.no>#1/1
X-Deja-AN: 281733646
References: <m2pvp2w8mw.fsf@KludgeUnix.com>
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Organization: Naggum Software; +47 8800 8879; http://www.naggum.no
Newsgroups: comp.lang.lisp


(I'll have to go look up the stuff in Norvig: PAIP before I answer the
application-level part, but the language part is easy.)

* Steve Gonedes
| (let ((this '(a b c)))
|   (progv '(a b c) '(+ 1 2)
|     (eval this)))
| 
| Is an error.  I think I might know why, but some clarification would be
| really great.

simply put: because a symbol has both a function and a value slot, used in
very distinct contexts.  e.g., (a a) is a call to the function binding of
`a' with the variable binding of `a' as the only argument.  you can
establish both with a form like this:

    (flet ((a (x) (* x x)))
      (let ((a 7))
	(values #'a a (a a))))

| These all result in error:
| 
| (progv '(a b c) '(list 1 2)
|   (eval (a b c)))

you're trying to use `a' as a function, which it (probably) isn't.  also,
you would send the _result_ of this call to `eval'.  if `a' actually was a
function of two arguments, this would probably be even harder to debug.

| (progv '(a b c) '(+ 1 2)
|   (eval '(a b c)))

again, you're trying to use `a' as a function, but this is the correct use
of `progv': the variable bindings for `b' and `c' are in effect
dynamically.

| (progv '(a b c) '(list 1 2)
|   (a b c))

again, calling `a' as a function, but at a very different time than above.

| and of course the following work fine:
| 
| (progv '(a b c) '(values 1 2)
|   (eval (list a b c)))

you capture the variable binding of `a' in this case.

| (progv '(a b c) '(values 1 2)
|   (funcall a b c))

and here you call `a' directly.

| Just what is so special about LIST (or cons)?

`eval' takes a list as argument.

| Why aren't the symbols bound within the PROGV?

oh, they are.  you're just not referring the proper binding.

also try

    (progv '(a b c) '(1 2 3)
      (eval '(list a b c)))

| I am somewhat regretful for having to resort to actually asking someone
| for help, as I usually come up with the solution - however long that may
| take (usually very long).  I have been trying various things for several
| days and can't seem to give it a rest and just use apply or what have
| you.

sometimes, asking for help is the _only_ right thing.  usually, however,
it's more important to establish what you already know before you ask for
help, or others will have a hard time actually helping you.  in the process
of establishing what I know and what I can't figure out, I have often found
the answer myself, but it's somewhat easier for me to do this when I'm
writing a request for help or (customer) support that I subsequently don't
need to send.  if I also felt that asking for help would be bad, I wouldn't
even try this venue.

| One more question (the nerve of this kid, huh?), why can't you apply the
| special DEFUN, this isn't a macro is it (probably implementation
| specific)?

DEFUN is a macro.

#\Erik
-- 
if you think this year is "97", _you_ are not "year 2000 compliant".

see http://www.naggum.no/emacs/ for Emacs-20-related material.