[racket-dev] syntax property duplication

On Wed, Sep 7, 2011 at 1:37 PM, John Clements <clements at brinckerhoff.org> wrote:
>
> On Sep 6, 2011, at 8:12 PM, Sam Tobin-Hochstadt wrote:
>
>> On Tue, Sep 6, 2011 at 8:05 PM, John Clements <clements at brinckerhoff.org> wrote:
>>>
>>> On Aug 18, 2011, at 5:32 AM, Sam Tobin-Hochstadt wrote:
>>>
>>>>
>>>> Yes, I understand why this happens.  As I see it, there are a few possibilities:
>>>>
>>>> 1. The expander should check for duplicates, in some fashion.
>>>> 2. This idiom is problematic, in the case where `stx' is both the
>>>> input and used for the syntax properties of the output.
>>>> 3. Macros may freely duplicate syntax properties.
>>>>
>>>> All of these have drawbacks, but (3), which you are suggesting, means
>>>> either that syntax properties can only be used to specify idempotent
>>>> information or that the non-idempotent ones need to have some
>>>> *explicit* means by which equal elements can be distinguished, which
>>>> must be part of the API of that syntax property.
>>>>
>>>> If we think this is how syntax properties ought to work, then we
>>>> should add something to the documentation making this clear, and we
>>>> should recognize that it's a limitation.
>>>
>>> Would syntax-property guards solve this problem?
>>
>> I don't really see how.  What are you thinking of?
>
> A guard could keep track of duplicates.  Keep in mind that syntax-property guards are purely a > figment of my imagination.

What, in your imagination, is a syntax-property guard?

--Carl