[racket-dev] syntax property duplication

On Sep 7, 2011, at 10:40 AM, Carl Eastlund wrote:

> 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?

I imagine that when the key involved in the property is a special guarded-syntax-property value, then an associated procedure gets to decide (approve/reject, substitute an alternate value?) whether a value should be used.

If I understand the problem, the obvious danger in an idea such as this is opening a "back door" into expansion time.

Apologies if I've misunderstood the issue.

John

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