pure macros

[jesse99]

It would be slick if macros could match a pure_expr. Stuff like logging macros could use a pure_expr to ensure that the code behaves the same way with logging on or off.

[graydon]

Purity checking happens much later than syntax expansion. We could force the result of a macro to be pure by wrapping it in a suitable function and passing it to some context that requires purity, but in general this is not a syntactic issue. Closing WONTFIX.