Re: global constants (was: re-using comma ...)

I think we are not quite on the same page. Here is what I mean:

(defparameter ag 1)
(defparameter bg 2)
(defconstant  cg 3)

(defun myfun (b)
   (declare (-double-) b)
   (let ((bg b))
     (+ ag bg cg) ))

(dhc-make ()
           myfun)

This will not compile in Lush, because the compiler cannot
create code that would access global variables at runtime.

But often all you want is using a symbol for a constant
value. So you may add (libload "libc/constants") and change
myfun to read

(defun myfun (b)
   (declare (-double-) b)
   (let ((bg b))
     (+ @ag bg @cg) ))

The @-macro character fetches the value of ag and cg,
respectively, at compile time (or at runtime if you don't
compile). All I'm saying is: that is something the
compiler might as well do, we don't need to use a special
macro character for this. E.g., here is how looks after a
little modification of the compiler when compiling the
above code:

? (libload "compiling-constants.lsh")
     [compiling-constants.lsh]
Preprocessing and parsing myfun ...
*** Warning: Treating global variable 'ag' as constant.
***          Declare with 'defconstant' to eliminate this warning.
Generating C for myfun ...
gcc -DHAVE_CONFIG_H   -std=c99 -DNO_DEBUG -Wall -O3 -march=i686 -mmmx 
-msse -pthread -I /u/juenglin/include 
-I/u/juenglin/projects/lush2-tinyat/include -I/usr/include/freetype2 -c 
/u/juenglin/src/lsh/C/compiling_constants.c -o 
/u/juenglin/src/lsh/C/i686-pc-linux-gnu/compiling_constants.o
= "/u/juenglin/src/lsh/compiling-constants.lsh"
? (myfun 2)
= 6
? (myfun 12)
= 16
?

Note the warning, we may turn it into an error if desirable.

Accessing global lisp variables from compiled code is a
different matter. I think it's possible but requires much
more work.

So, are there any good reasons for keeping @ or @@?

Ralf





On Tue, 21 Oct 2008, Yann LeCun wrote:

> Reading global variables would be nice, but @ and @@
> serve a different purpose. They are more akin to #define
> in C/C++.
>
> Reading global vars from compiled code is actually not easy.
> At least, I have no idea how to implement that cleanly.
>
>  -- Yann
>
>
> On Tuesday 21 October 2008, Ralf Juengling wrote:
>> On Tue, 21 Oct 2008, Yann LeCun wrote:
>>> I'd try to avoid non-backward compatible changes that
>>> make the documentation more complicated.
>>
>> Ok. I think we should at least deprecate the current use of
>> @ and fix the compiler so that it sees lisp global variables.
>> That will also simplify the documentation. ;-)
>>
>> Ralf
>>
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