[racket-dev] syntax property duplication

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?

John


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