Re: Lexical binding

[Christian Ohler <ohler@gnu.org>]

On 3/04/11 22:26, Juanma Barranquero wrote:
> On Sun, Apr 3, 2011 at 14:05, Christian Ohler<ohler@gnu.org>  wrote:
>
>> In loop destructuring, you can use nil for fields that you want to ignore:
>>
>>      (loop for (nil width . nil) in bs-attributes-list
>>            if (numberp width) sum width)
>
> Yes, but it is less informative.
>
>> If you want to keep the ignored fields named, you could do something like
>>
>>      (loop for (name width . rest) in bs-attributes-list
>>            do (progn name rest) ; ignore
>>            if (numberp width) sum width)
>
> And this is just ugly.

How about:

     (loop for (name width . rest) in bs-attributes-list
           do (ignore name rest)
           if (numberp width) sum width)


> Anyway, the point is that (elisp)11.9.5.1 "Converting a package to use
> lexical scoping" says:
>
>       To silence byte-compiler warnings about unused variables, just use a
>    variable name that start with an underscore, which the byte-compiler
>    interpret as an indication that this is a variable known not to be used.
>
> but this exception is quite less useful if assignments produce the
> "variable `_x' not left unused" warning.

IIRC, Common Lisp distinguishes between (declare (ignore ...)) and 
(declare (ignorable ...)).  The latter is useful for macros that 
introduce bindings that may or may not be used.  This doesn't really 
help with `loop', though, since it shouldn't declare its bindings ignorable.

I guess Emacs Lisp's new underscore syntax is the equivalent of (declare 
(ignore ...)), with no way to (declare (ignorable ...)).  If this is the 
case, then I agree it's not quite expressive enough, and requires idioms 
like the above (with many macros, not just `loop').

Christian.