Editing Scheme (programming language) (section)

===Hygienic macros===
{{Main|Hygienic macro}}
In the R5RS standard and also in later reports, the syntax of Scheme can easily be extended via the macro system. The R5RS standard introduced a powerful hygienic macro system that allows the programmer to add new syntactic constructs to the language using a simple [[pattern matching]] sublanguage (R5RS sec 4.3).<ref name="r5rs"/>  Prior to this, the hygienic macro system had been relegated to an appendix of the R4RS standard, as a "high level" system alongside a "low level" macro system, both of which were treated as extensions to Scheme rather than an essential part of the language.<ref name="r4rs">{{Cite journal |year=1991 |title=Revised<sup>4</sup> Report on the Algorithmic Language Scheme |url=http://www.cs.indiana.edu/scheme-repository/R4RS/r4rs_toc.html |journal=ACM Lisp Pointers |volume=4 |issue=3 |pages=1–55 |access-date=2012-08-09 |editor=William Clinger and Jonathan Rees}}</ref>

Implementations of the hygienic macro system, also called <code>syntax-rules</code>, are required to respect the lexical scoping of the rest of the language. 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. R6RS specifies a more sophisticated transformation system, <code>syntax-case</code>, which has been available as a language extension to R5RS Scheme for some time.

<syntaxhighlight lang="Scheme">
;; Define a macro to implement a variant of "if" with a multi-expression
;; true branch and no false branch.
(define-syntax when
  (syntax-rules ()
    ((when pred exp exps ...)
      (if pred (begin exp exps ...)))))
</syntaxhighlight>

Invocations of macros and procedures bear a close resemblance—both are s-expressions—but they are treated differently. When the compiler encounters an s-expression in the program, it first checks to see if the symbol is defined as a syntactic keyword within the current lexical scope. If so, it then attempts to expand the macro, treating the items in the tail of the s-expression as arguments without compiling code to evaluate them, and this process is repeated recursively until no macro invocations remain. If it is not a syntactic keyword, the compiler compiles code to evaluate the arguments in the tail of the s-expression and then to evaluate the variable represented by the symbol at the head of the s-expression and call it as a procedure with the evaluated tail expressions passed as arguments to it.

Most Scheme implementations also provide additional macro systems. Among popular ones are [[Hygienic macro#syntactic closures|syntactic closures]], [[Hygienic macro#explicit renaming|explicit renaming macros]] and <code>define-macro</code>, a non-hygienic macro system similar to <code>defmacro</code> system provided in [[Common Lisp]].

The inability to specify whether or not a macro is hygienic is one of the shortcomings of the macro system. Alternative models for expansion such as scope sets provide a potential solution.<ref>{{Cite book |last=Flatt |first=Matthew |title=Proceedings of the 43rd Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages |year=2016 |isbn=978-1-4503-3549-2 |pages=705–717 |chapter=Binding as sets of scopes |doi=10.1145/2837614.2837620 |s2cid=15401805}}</ref>