From: pjb@informatimago.com (Pascal J. Bourguignon)
To: help-gnu-emacs@gnu.org
Subject: Re: Creating a list
Date: Thu, 19 Nov 2009 14:59:38 +0100 [thread overview]
Message-ID: <873a4aipit.fsf@galatea.local> (raw)
In-Reply-To: 873a4afyi0.fsf@Traian.DecebalComp
Cecil Westerhof <Cecil@decebal.nl> writes:
> David Kastrup <dak@gnu.org> writes:
>
>>>>> At the moment I create a list with:
>>>>> (setq ret-val (cons total-amount (cons current-amount ())))
>>>>> I thought about doing it with:
>>>>> (setq ret-val (cons total-amount '(current-amount)))
>>>>>
>>>>> But then the second value is the string current-amount
>>>>
>>>> Wrong. The _symbol_ current-amount.
>>>
>>> When evaluating I got:
>>> (1570378.2570192777 current-amount)
>>> That is why I thought I got the string.
>>
>> A string would have quote marks around it.
>
> Off course.
>
> But I have a few questions on my mind.
> - Why is the first shown as value and the second as _symbol_?
Both floating point numbers and symbols ARE VALUES!
The first object is a floating point number, the second object is a
symbol. Therefore it shows a floating point number and a symbol.
The rules of evaluation of lisp include:
(defun eval (expression)
(cond
((symbolp expression) (symbol-value expression)) ; symbols evaluate to their value
((atom expression) expression)) ; atoms are self evaluating
(t (case (first expression)
((quote) (second expression)) ; (quote x) returns x unchanged.
; ... other special operators
(else
(cond
((and (listp (first expression))
(eq 'lambda (first (first expression))))
;; ... process ((lambda (...) ...) ...)
)
((macro-function (first expression))
;; ... expand the macro call
)
((and (symbolp (first expression)) (fboundp (first expression)))
(apply (symbol-function (first expression))
(mapcar (function eval) (rest expression)) ; evaluate the arguments
)))))))
When you evaluate (cons current-amount ()), since cons is a symbol
that is fbound, the arguments are evaluated:
since current-amount is a symbol, its symbol-value is returned.
since () is a symbol, its symbol-value is returned (it's nil).
the function #<subr cons> is called with the above values.
When you evalute (quote (current-value)), the list (current-value) is
returned. The evaluation doesn't even look at what's inside the list!
> - current-amount is a local variable. How is it possible that the symbol
> is still defined?
You exist. How is it possible your name still exists?
--
__Pascal Bourguignon__
next prev parent reply other threads:[~2009-11-19 13:59 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-19 7:38 Creating a list Cecil Westerhof
2009-11-19 8:35 ` David Kastrup
2009-11-19 10:59 ` Cecil Westerhof
2009-11-19 11:41 ` David Kastrup
2009-11-19 13:13 ` Cecil Westerhof
2009-11-19 13:47 ` Barry Margolin
2009-11-19 15:11 ` Cecil Westerhof
2009-11-19 19:26 ` Pascal J. Bourguignon
2009-11-19 13:57 ` David Kastrup
2009-11-19 15:14 ` Cecil Westerhof
2009-11-19 13:59 ` Pascal J. Bourguignon [this message]
2009-11-19 11:59 ` Gnus: Some new mails are automatically marked as 'O' Wang Lei
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