Re: Basic Emacs Lisp question

[Tim X <timx@nospam.dev.null>]

Giorgos Keramidas <keramida@ceid.upatras.gr> writes:

> On Tue, 29 Apr 2008 20:56:17 +0300, Giorgos Keramidas <keramida@ceid.upatras.gr> wrote:
>> On Tue, 29 Apr 2008 17:49:19 +0200, Matthias Pfeifer <pfemat@web.de> wrote:
>>> Hello,
>>>
>>> What is the difference between
>>>
>>> (list 0 nil -1)
>>>
>>> and
>>>
>>> '(0 nil -1)
>>
>> In Common Lisp (list 0 nil -1) is required to 'cons' a new list every
>> time it is called.  Quoting the list as in '(0 nil -1) is not required
>> to build a new list.  In fact, in compiled code it may reuse the same
>> static object over and over again.
>
> Reading my own post reveals that I may have been too terse.  To clarify
> the point I was trying to make, here's a small test in Common Lisp, and
> the equivalent test in Emacs Lisp.
>
> 1. Common Lisp test
> -------------------
>
> * Save the following Lisp code to a file called "foo.lisp":
>
>   (defun foo-quoted ()
>     '(0 nil -1))
>
>   (defun foo-list ()
>     (list 0 nil -1))
>
> * Then compile the file, and load it.  Here's the output from loading
>   the compiled file in SBCL:
>
>   CL-USER> (compile-file "foo")
>
>   ; compiling file "/home/keramida/foo.lisp" (written 30 APR 2008 01:48:02 AM):
>   ; compiling (DEFUN FOO-QUOTED ...)
>   ; compiling (DEFUN FOO-LIST ...)
>
>   ; /home/keramida/foo.fasl written
>   ; compilation finished in 0:00:00
>   #P"/home/keramida/foo.fasl"
>   NIL
>   NIL
>   CL-USER> (load "foo")          ;; This actually loads "foo.fasl" in SBCL.
>   T
>   CL-USER>
>
> * Every time the `foo-quoted' function runs it returns exactly the same
>   compiled object.  The object returned by separate calls to
>   `foo-quoted' is all of EQ, EQL and EQUAL to any previous call, as you
>   can see in:
>
>   CL-USER> (let ((one-list (foo-quoted))
>                  (another-list (foo-quoted)))
>              (mapcar (lambda (test)
>                        (funcall test one-list another-list))
>                      (list #'eq #'eql #'equal)))
>   (T T T)
>   CL-USER>
>
> * In contrast, the object returned by the `foo-list' function is a newly
>   CONS-ed list every time the function runs:
>
>   CL-USER> (let ((one-list (foo-list))
>                  (another-list (foo-list)))
>              (mapcar (lambda (test)
>                        (funcall test one-list another-list))
>                      (list #'eq #'eql #'equal)))
>   (NIL NIL T)
>   CL-USER>
>
> The lists returned by `foo-list' are EQUAL, but they are neither EQ nor
> EQL to each other.  They are created from scratch by allocating new
> storage for the value of the expression every time the `foo-list'
> function is called.
>
> 2. Emacs Lisp test
> ------------------
>
> * Save the same two functions in a file called "foo.el".
>
> * Fire up Emacs, and byte-compile the file by typing
>
>   M-x byte-compile-file RET foo.el RET
>
> * Load the byte-compiled file by typing
>
>   M-x load-file RET foo.elc RET
>
> * Now evaluate the same two LET forms in your scratch buffer, by pasting
>   them in the buffer and typing `C-x C-e' after each expression.
>
>   Emacs Lisp should also evaluate them as:
>
>   (let ((one-list (foo-quoted))
>         (another-list (foo-quoted)))
>     (mapcar (lambda (test)
>               (funcall test one-list another-list))
>             (list #'eq #'eql #'equal)))
>   => (t t t)
>
>   (let ((one-list (foo-list))
>         (another-list (foo-list)))
>     (mapcar (lambda (test)
>               (funcall test one-list another-list))
>             (list #'eq #'eql #'equal)))
>   => (nil nil t)
>
> I hope this makes what I initially wrote a bit easier to grasp :-)
>

I think you expressed the difference quite clearly. In another post, I
tried to point out a possible trap that many fall into - attempting to
modify the list returned by the quoted version. This is why I tend to
think of '(a b c) more like a constant, while (list a b c) is able to be
modified with expected results. 

Tim


-- 
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