Define-syntax

Hygienic macros are macros whose expansion is guaranteed not to cause the accidental capture of identifiers. They are a feature of programming languages such as Scheme,Dylan and Rust. The general problem of accidental capture was well known within the Lisp community prior to the introduction of hygienic macros. Macro writers would use language features that would generate unique identifiers (e.g., gensym) or use obfuscated identifiers in order to avoid the problem. Hygienic macros are a programmatic solution to the capture problem that is integrated into the macro expander itself. The term "hygiene" was coined in Kohlbecker et al.'s 1986 paper that introduced hygienic macro expansion, inspired by the terminology used in mathematics.

In programming languages that have non-hygienic macro systems, it is possible for existing variable bindings to be hidden from a macro by variable bindings that are created during its expansion. In C, this problem can be illustrated by the following fragment:

Running the above through the C preprocessor produces:

The variable a declared in the top scope is shadowed by the a variable in the macro, which introduces a new scope. As a result, it is never altered by the execution of the program, as the output of the compiled program shows:

The simplest solution is to give the macros variables names that do not conflict with any variable in the current program:

Until a variable named INCIa is created, this solution produces the correct output:

The problem is solved for the current program, but this solution is not robust. The variables used inside the macro and those in the rest of the program have to be kept in sync by the programmer. Specifically, using the macro INCI on a variable INCIa is going to fail in the same way that the original macro failed on a variable a.

The "hygiene problem" can extend beyond variable bindings. Consider this Common Lisp macro:

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