Due to the deficiencies of a purely syntax-rules based macro system, low-level macro systems have also been proposed and implemented for Scheme.
12.
Others have proposed alternative uses of macros, such as anaphoric macros in macro systems that are unhygienic or allow selective unhygienic transformation.
13.
Implementations of the hygienic macro system, also called syntax-rules, are required to respect the lexical scoping of the rest of the language.
14.
Due to the deficiencies of a purely syntax-rules based macro system, low-level macro systems have also been proposed and implemented for Scheme.
15.
In cases where accidental capture is desired, some systems allow the programmer to explicitly violate the hygiene mechanisms of the macro system.
16.
The HLA macro system is actually a subset of a larger feature known as the HLA " Compile-Time Language " ( CTL ).
17.
Indeed, it is fair to say that the macro system is a carefully tuned application programming interface ( API ) for compiler extensions.
18.
On the other hand, hygienic macro systems preserve the lexical scoping of all identifiers ( such as " if " and " not " ) automatically.
19.
In contrast to Lisp's macro system and Perl's BEGIN blocks, which may contain general computations, C macros are merely string replacements and do not require code execution.
20.
This is assured by special naming and scoping rules for macro expansion, and avoids common programming errors that can occur in the macro systems of other programming languages.
