Hygienic Macro - Strategies

Strategies

In some languages such as Common Lisp, Scheme and others of the Lisp language family, macros provide a powerful means of extending the language. Here the lack of hygiene in conventional macros is resolved by several strategies.

Obfuscation

If the programmer needs to use temporary storage during the expansion of a macro, he can use one with an unusual name and hope that the same name will never be used in a program that uses his macro. Of course any programmer knowing of gensym won't do this. (See next point)

Temporary symbol creation

In some programming languages, it is possible for a new variable name, or symbol, to be generated and bound to a temporary location. The language processing system ensures that this never clashes with another name or location in the execution environment. The responsibility for choosing to use this feature within the body of a macro definition is left to the programmer. This method was used in MacLisp, where a function named gensym could be used to generate a new symbol name. Similar functions (usually named gensym as well) exist in many Lisp-like languages, including the widely implemented Common Lisp standard.

Read-time Uninterned Symbol

This is similar to the first solution in that a single name is shared by multiple expansions of the same macro. Unlike an unusual name, however, a read time uninterned symbol is used (denoted by the #: notation), for which it is impossible to occur outside of the macro.

Packages

Instead of an unusual name or an uninterned symbol, the macro simply uses a private symbol from the package in which the macro is defined. The symbol will not accidentally occur in user code. User code would have to reach inside the package using the double colon (::) notation to give itself permission to use the private symbol, for instance cool-macros::secret-sym. At that point, the issue of accidental lack of hygiene is moot. Thus the Lisp package system provide a viable, complete solution to the macro hygiene problem, which can be regarded as an instance of name clashing.

Hygienic transformation

The processor responsible for transforming the patterns of the input form into an output form detects symbol clashes and resolves them by temporarily changing the names of symbols. This kind of processing is supported by Scheme's let-syntax and define-syntax macro creation systems. The basic strategy is to identify bindings in the macro definition and replace those names with gensyms, and to identify free variables in the macro definition and make sure those names are looked up in the scope of the macro definition instead of the scope where the macro was used.