But you can do this without S-expressions. DLang, for example, has free-form macros that they weirdly call "mixin" (as in, mixing in some text or declarations

But you can do this without S-expressions. DLang, for example, has free-form macros that they weirdly call "mixin" (as in, mixing in some text or declarations into the AST, syntax-wise). Each mixin must be a valid AST subtree on its own, which gives the same guarantee that paren matching prior to macro expansion gives you. Then, the compiler can interleave: parsing -> evaluating mixin strings -> resuming parsing of the mixin subtrees -> evaluating the deeper mixin strings -> ... You get the power of Lisp macros, but without the Lisp syntax that is unattractive to many.