[Openmcl-devel] User contributions

[Ron Garret]

On Jun 13, 2009, at 6:03 PM, Ron Garret wrote:

>
> On Jun 13, 2009, at 5:58 PM, Ron Garret wrote:
>
>>
>> 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.
>>
>
> This seems to mostly work:
>
> (defconstant +guardian+ '#.(gensym))
>

But I think this is the Right Answer:

(defmacro defguardian (name)
   `(defconstant ,name (if (boundp ',name) ,name (gensym))))

Or if you're really being paranoid:

(defmacro defguardian (name)
   `(defconstant ,name (if (boundp ',name) ,name '#.(gensym))))

rg