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 | | From: | Ulf Wikstr?m | | Subject: | Newbie question | | Date: | 17 Jan 2005 01:53:29 -0800 |
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 | Hello!
Is (cons a b) and '(a . b) not the same thing?
I want to define a function bind1 like this:
(defun bind1 (fun arg1)
#'(lambda (&rest rest)
(apply fun '(arg1 . rest))))
When i try to do:
(funcall (bind1 #'+ 2) 3)
i get the following error message:
*** - APPLY: argument list given to + is dotted (terminated by REST)
This code works, however:
(defun bind1 (fun arg1)
#'(lambda (&rest rest)
(apply fun (cons arg1 rest))))
I really can't see the difference.. Am I missing something obvious?
Thanks!
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| | From: | David Sletten | | Subject: | Re: Newbie question | | Date: | Mon, 17 Jan 2005 10:17:56 GMT |
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| Ulf Wikstr?m wrote:
> Hello!
> Is (cons a b) and '(a . b) not the same thing?
>
> I want to define a function bind1 like this:
>
> (defun bind1 (fun arg1)
> #'(lambda (&rest rest)
> (apply fun '(arg1 . rest))))
>
> When i try to do:
>
> (funcall (bind1 #'+ 2) 3)
>
> i get the following error message:
>
> *** - APPLY: argument list given to + is dotted (terminated by REST)
>
> This code works, however:
>
> (defun bind1 (fun arg1)
> #'(lambda (&rest rest)
> (apply fun (cons arg1 rest))))
>
> I really can't see the difference.. Am I missing something obvious?
>
> Thanks!
Yes, you are missing something obvious. Here's what it is...
In Lisp a single-quote is a syntactic shortcut for the QUOTE special
operator:
http://www.lispworks.com/documentation/HyperSpec/Body/s_quote.htm
'(a b c) becomes (quote (a b c)). The QUOTE operator simply returns its
argument without evaluation:
'(a b c) => (A B C)
(quote (a b c)) => (A B C)
Your example '(a . b) evaluates neither A nor B:
'(a . b) => (A . B)
However, CONS is a function, so its arguments are evaluated before it
gets called:
(let ((a 1)
(b 2))
(cons a b))
=> (1 . 2)
Here, the _value_ of the variable A and the _value_ of the variable B
are sent to CONS, so your analogy is not quite correct. Instead, '(a .
b) is equivalent to (cons 'a 'b). Here the _symbol_ A and the _symbol_ B
are CONSed.
In your first function definition, you are trying to APPLY #'+ to the
CONS (ARG1 . REST) consisting of the _symbols_ ARG1 and REST, not the
_values_ of the parameters named ARG1 and REST. In your second
definition, you correctly evaluate these parameters, and your code works.
David Sletten
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| | From: | Frode Vatvedt Fjeld | | Subject: | Re: Newbie question | | Date: | Mon, 17 Jan 2005 11:06:51 +0100 |
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| ulfwikstrom@netscape.net (Ulf Wikstr?m) writes:
> Is (cons a b) and '(a . b) not the same thing?
No, the former creates a new cons-cell whose car and cdr are the
values of the variables a and b, while the second form returns the
same (constant) object which is a cons-cell whose car and cdr are the
symbols a and b. Just try to evaluate the forms to see the difference.
> I want to define a function bind1 like this:
>
> (defun bind1 (fun arg1)
> #'(lambda (&rest rest)
> (apply fun '(arg1 . rest))))
Try this:
(defun bind1 (fun arg1)
(lambda (&rest rest)
(apply fun arg1 rest)))
--
Frode Vatvedt Fjeld
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