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Concept
Hygienic macro
Summary
In computer science, 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, Rust, Nim, and Julia. The general problem of accidental capture was well known in the Lisp community before the introduction of hygienic macros. Macro writers would use language features that would generate unique identifiers (e.g., gensym) or use obfuscated identifiers to avoid the problem. Hygienic macros are a programmatic solution to the capture problem that is integrated into the macro expander. The term "hygiene" was coined in Kohlbecker et al.'s 1986 paper that introduced hygienic macro expansion, inspired by 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:
#define INCI(i) do { int a=0; ++i; } while (0)
int main(void)
{
int a = 4, b = 8;
INCI(a);
INCI(b);
printf("a is now %d, b is now %d\n", a, b);
return 0;
}
Running the above through the C preprocessor produces:
int main(void)
{
int a = 4, b = 8;
do { int a = 0; ++a; } while (0);
do { int a = 0; ++b; } while (0);
printf("a is now %d, b is now %d\n", a, b);
return 0;
}
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, a is never altered by the execution of the program, as the output of the compiled program shows:
a is now 4, b is now 9
The hygiene problem can extend beyond variable bindings. Consider this Common Lisp macro:
(defmacro my-unless (condition &body body)
`(if (not ,condition)
(progn
@body)))
While there are no references to variables in this macro, it assumes the symbols "if", "not", and "progn" are all bound to their usual definitions in the standard library.
Official source
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