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Roughly, hygienic macro expansion is desirable for the same reason as lexical scope: both enable local reasoning about binding so that program fragments compose reliably. The analogy suggests specifying hygienic macro expansion as a kind of translation into lexical-scope machinery. In that view, variables must be renamed to match the mechanisms of lexical scope as macro expansion proceeds. A specification of hygiene in terms of renaming accommodates simple binding forms well, but it becomes unwieldy for recursive definition contexts (Flatt et al. 2012, section 3.8), especially for contexts that allow a mixture of macro and non-macro definitions. The renaming approach is also difficult to implement compactly and efficiently in a macro system that supports “hygiene bending” operations, such as datum->syntax, because a history of renamings must be recorded for replay on an arbitrary symbol.
In a new macro expander for Racket, we discard the renaming approach and start with a generalized idea of macro scope, where lexical scope is just a special case of macro scope when applied to a language without macros. Roughly, every binding form and macro expansion creates a scope, and each fragment of syntax acquires a set of scopes that determines binding of identifiers within the fragment. In a language without macros, each scope set is identifiable by a single innermost scope. In a language with macros, identifiers acquire scope sets that overlap in more general ways.
