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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