On Feb 9, 2010, at 8:42 AM, Aleksandar Matijaca wrote: > Why is it then that this fails > > (reverse (1 2 3 4)) > > Can't reverse deal with a non-quoted list? Or is it that functions can only deal > with quoted "things" (sorry, I am missing here the exact English language expression > as to how to express myself)... 1. Lisp functions receive all their arguments already evaluated. What is the evaluation of (1 2 3 4)? hint: the number 1 is not a function. When lispworks, or any other common lisp, tries to evaluate (reverse (1 2 3 4)) it first must evaluate (1 2 3 4) - not (list 1 2 3 4), nor (quote 1 2 3 4) which is what '(1 2 3 4) is a shorthand for. Since the number 1 is not a function, you get and error, specifically "Illegal argument in functor position: 1 in (1 2 3 4)." LispWorks is even nice enough to tell you what exactly in your input is *not* a function, here, the number 1. 2. You definitely need to read, thoroughly and completely, an introduction to lisp. This mailing list is intended for questions specific to LispWorks Common Lisp, not general newbie questions about the language. For that, try reading an introductory text, or the usenet group comp.lang.lisp. For a good, free, online intro, try: <http://www.gigamonkeys.com/book/> warmest regards, Ralph Raffael Cavallaro raffaelcavallaro@me.com
Hi there,
I have a question --
if I write in REPL
(reverse '(1 2 3 4))
I get a reasonable result -- that is, reverse reverses a quoted list in this case...Why is it then that this fails
(reverse (1 2 3 4))
Can't reverse deal with a non-quoted list? Or is it that functions can only dealwith quoted "things" (sorry, I am missing here the exact English language expressionas to how to express myself)...
Thanks, Alex.
On Tue, Feb 9, 2010 at 8:42 AM, Aleksandar Matijaca <amatijaca@gmail.com> wrote: > Hi there, > I have a question -- > if I write in REPL > (reverse '(1 2 3 4)) > I get a reasonable result -- that is, reverse reverses a quoted list in this > case... > Why is it then that this fails > (reverse (1 2 3 4)) > Can't reverse deal with a non-quoted list? Or is it that functions can only > deal > with quoted "things" (sorry, I am missing here the exact English language > expression > as to how to express myself)... > Thanks, Alex. HI Alex, This is a more fundamental question about Lisp, and not LispWorks specific. As such, you might be better off finding an introductory book about Lisp. Some popular ones include Practical Common Lisp [1], On Lisp [2], ANSI Common Lisp [3]. The first two of these are freely available online, and there are lots of other resources out there too. Now, toward your original question… As you saw, (reverse '(1 2 3 4)) produces (4 3 2 1). What do you expect (reverse (reverse '(1 2 3 4))) to return? I hope that you expect (1 2 3 4) and not ((1 2 3 4) reverse). Reverse accepts a single list l and returns a new list containing the elements of l in reverse order. When (reverse <form>) appears in a program, the <form> is evaluated to produce a value, which should be a list, and then this value is passed to reverse. Whether the <form> is a quoted list or something else doesn't matter, so long as it produces a list. The reason that (reverse (1 2 3 4)) does work, is that here <form> is (1 2 3 4), and to evaluate (1 2 3 4) means to find the function associated with 1 (and there isn't one) to evaluate 2, 3, and 4, to produce the values 2, 3, and 4, and then to call the function associated with 1 with the values 2, 3, and 4. Since there is no such function, this will fail. If you've tried this in the REPL, you'll see something like: CL-USER 47 > (1 2 3 4) Error: Illegal argument in functor position: 1 in (1 2 3 4). … (reverse '(1 2 3 4)) works because the value of '(1 2 3 4) is a list. The value of (list 1 2 3 4) is also a list, so (reverse (list 1 2 3 4)) will work for you too. Similarly with (reverse (reverse '(1 2 3 4))) and (reverse (reverse (list 1 2 3 4))). But I think you need to find some introductory Lisp material before going much farther. Cheers, //JT [1] http://www.gigamonkeys.com/book/ [2] http://www.paulgraham.com/onlisp.html [3] http://www.paulgraham.com/acl.html -- Joshua Taylor, http://www.cs.rpi.edu/~tayloj/
Am 09.02.2010 um 14:42 schrieb Aleksandar Matijaca: > Hi there, > > I have a question -- > > if I write in REPL > > (reverse '(1 2 3 4)) > > I get a reasonable result -- that is, reverse reverses a quoted list in this case... > Why is it then that this fails > > (reverse (1 2 3 4)) > > Can't reverse deal with a non-quoted list? Or is it that functions can only deal > with quoted "things" (sorry, I am missing here the exact English language expression > as to how to express myself)... > > Thanks, Alex. > Here is another book introducing some basic Common Lisp: http://www.cs.cmu.edu/~dst/LispBook/ You can download the PDF. Practical Common Lisp, by Peter Seibel, is a bit more advanced and also more practical. While reading these books it is very useful to try out the examples and exercises directly with the LispWorks editor and listener. Regards, Rainer Joswig Rainer Joswig, Hamburg, Germany http://lispm.dyndns.org/ mailto:joswig@lisp.de