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From: Erik Naggum <erik@naggum.net>
Newsgroups: comp.lang.lisp
Subject: Re: Trouble with labels
Date: 11 Apr 2001 12:58:10 +0000
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* iscaris@hotmail.com
> How can I make a function defined in labels refer to another function
> defined in the same labels body?

  The same way you refer to functions everywhere else in Common Lisp.

  I honestly do not understand why you are not using a Scheme system to
  learn Scheme when you don't know Common Lisp well enough to know how to
  refer to functions.  Perhaps you think that Scheme is a Lisp and Common
  Lisp is a Lisp, so it should work?  Just because two human languages have
  Indo-European roots doesn't mean that they can be interchanged if you
  want to learn one of them, either.

> For example (this is from SICP page 223):
> (defun make-account (balance)
>   (labels ((withdraw (amount)
>                      (if (>= balance amount)
>                          (progn
>                            (setf balance (- balance amount))
>                            balance)
>                        (format t "Insufficient funds")))
>            (deposit (amount)
>                     (setf balance (+ balance amount))
>                     balance)
>            (dispatch (m)
>                      (cond
>                       ((equal m 'withdraw) withdraw)
>                       ((equal m 'deposit) deposit)
>                       (t (format t "Unknown request -- MAKE-ACCOUNT")))))
>     dispatch))

  This is what object-oriented programming looks like in Scheme, with
  message-passing as the dispatch paradigm, using closures to hold the
  data.  If you want to understand what it does, stick to Scheme and learn
  how it works in Scheme.  Then see if the same concept, once understood,
  is applicable anywhere else and whether it may be grafted into any other
  system.  (IMNSHO, it neither can nor should be.)

  Scheme and Common Lisp are extremely different languages.  You should not
  assume that you can make Scheme code become Common Lisp code with a few
  minor changes.  For instance, set! in Scheme is defined not to have a
  useful value.  setf in Common Lisp is defined to have the value of the
  last value.  This means that any Common Lisp programmer will look at your
  code and wonder why you're doing what you're doing, if there's something
  special going on.  And why use equal with symbols?  I briefly wondered
  whether that meant that m could be a string.  What use is writing out
  error messages that don't even say what's wrong?  This (among a host of
  other things) tells me that the whole example is designed to be used only
  as a simple toy to illustrate something, just as Scheme is.

  Here's how I would have written a similarly skeletal example in (real)
  Common Lisp:

(defclass account ()
  ((balance :initarg :deposit :initform 0 :accessor balance)))

(defmethod withdraw ((account account) amount)
  (decf (balance account) amount))

(defmethod deposit ((account account) amount)
  (incf (balance account) amount))

(defmethod withdraw :before ((account account) amount)
  (unless (<= amount (balance account))
    (error "~A has insufficient funds to withdraw ~A." account amount)))

#:Erik
-- 
  I found no peace in solitude.
  I found no chaos in catastrophe.
			-- :wumpscut: