[Openmcl-devel] User contributions

[Ron Garret]

On Jun 13, 2009, at 4:43 PM, Brian Mastenbrook wrote:

> On Jun 13, 2009, at 5:51 PM, Ron Garret wrote:
>
>> And just in case anyone actually tried this, it won't work.  The
>> reason is that I'm doing (essentially) this:
>>
>> (defconstant +guardian+ (gensym))
>>
>> as part of my iterators code.  But it turns out this is a broken  
>> idiom
>> because, as Gary points out in http://trac.clozure.com/ccl/ticket/ 
>> 539:
>>
>> The spec says:
>>
>> "If a defconstant form appears as a top level form, the compiler must
>> recognize that name names a constant variable. An implementation may
>> choose to evaluate the value-form at compile time, load time, or  
>> both.
>> Therefore, users must ensure that the initial-value can be evaluated
>> at compile time (regardless of whether or not references to name
>> appear in the file) and that it always evaluates to the same value."
>>
>>
>> I am open to suggestions on how to fix this because this leave me  
>> at a
>> bit of a loss.
>
> Use a symbol-macro instead:
>
> (macrolet ((define-guardian (name) `(define-symbol-macro ,name ', 
> (gensym))))
>   (define-guardian +guardian+))

That fails if you reload an uncompiled version of the file.

Ironically, using DEFVAR instead of DEFCONSTANT or DEFINE-SYMBOL-MACRO  
comes closest to doing the Right Thing -- but then you can't rely on  
the system to prohibit assignment and rebinding.

rg