Steel Bank Common Lisp

勇勇   <zhongzhaoyong@...> writes:

> ; compilation unit finished
> ;   caught 2 WARNING conditions
> "dave"
> CL-USER>
>
> it sucess to define a ,but why it displays "caught WARNING" ? do i do something wrong?

 "Catching a condition" is standard terminology.  You might look at the
corresponding sections in the Hyperspec or Kent Pitman's condition paper:
  http://www.nhplace.com/kent/Papers/Condition-Handling-2001.html

Nicolas

On Sun, 23 Mar 2008 08:40:39 +0800, 勇勇 wrote:

> hi,everyone:
>     
>     i am new to sbcl and lisp! when i use "setq" to define something in
>     emacs(slime+sbcl)
> SLIME 2006-04-20
> CL-USER> (setq a "dave")
> 
> ; in: LAMBDA NIL
> ;     (SETQ A "dave")
> ;
> ; caught WARNING:
> ;   undefined variable: A
> 
> ;
> ; caught WARNING:
> ;   This variable is undefined:
> ;     A
> ;
> ; compilation unit finished
> ;   caught 2 WARNING conditions
> "dave"
> CL-USER>
> 
> it sucess to define a ,but why it displays "caught WARNING" ? do i do
> something wrong?


Think about the name "SETF". It assigns a value to something. This 
"something" should be something it has seen before, or something you have 
declared before it.


CL-USER> (defparameter *a*)
*A*
CL-USER> (setf *a* "dave")
"dave"
CL-USER> *a*
"dave"
CL-USER> (let ((a nil))
           (setf a "dave")
           a)
"dave"
CL-USER> 


..this way, you can be sure that you are only working with things you 
have "declared that you want to exist". Take this for instance:


CL-USER> (let ((a nil))
           (setf a "dave")
           (setf *b* "huh?"))

; in: LAMBDA NIL
;     (SETF *B* "huh?")
; ==>
;   (SETQ *B* "huh?")
; 
; caught WARNING:
;   undefined variable: *B*

; 
; caught WARNING:
;   This variable is undefined:
;     *B*
; 
; compilation unit finished
;   caught 2 WARNING conditions
"huh?"
CL-USER> *b*
"huh?"


Suddenly *B* has popped up in the global environment without you having 
declared that it should exist (there). ...messy...

-- 
Lars Rune Nøstdal
http://nostdal.org/

勇勇   writes:
> hi,everyone:
>     
>     i am new to sbcl and lisp! when i use "setq" to define something in emacs(slime+sbcl)
> SLIME 2006-04-20
> CL-USER> (setq a "dave")
> 
> ; in: LAMBDA NIL
> ;     (SETQ A "dave")
> ; 
> ; caught WARNING:
> ;   undefined variable: A
> 
> ; 
> ; caught WARNING:
> ;   This variable is undefined:
> ;     A
> ; 
> ; compilation unit finished
> ;   caught 2 WARNING conditions
> "dave"
> CL-USER> 
> 
> it sucess to define a ,but why it displays "caught WARNING" ? do i do something wrong?


Yes, you did something wrong: you tried to assign an undefined variable.

The CL standard doesn't define what should happen when you assign a
variable that has not be defined.   It leaves it up to the
implementation, and each implementation may do something different.  



You can define a lexical variable with LET:

(let (a)
  (setq a "dave")
  (print a)
  (values))

but of course, the variable A won't be defined outside of it's lexical
scope.


Or you can define a global dynamic variable with DEFVAR or DEFPARAMETER:

(defvar *a*)
(setq *a* "dave")
(print *a*)


But there is no way provided by CL to define a global lexical
variable.  (It's possible to get something having the same effects
than a global lexical variable with symbol macros).



Note that since dynamic varibles are special, there is this convention
to name them surrounded by stars.   The problem is that once you've
declared a variable special (dynamic), any use of that variable will
be special:

(defvar s 0)

(defun f ()
  (print (list 'f '--> 's '= s))
  (values))

(defun g ()
  (let ((s 1))
     (f)
     (values)))

(g) prints:

(F --> S = 1) 


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/

"Debugging?  Klingons do not debug! Our software does not coddle the
weak."