Editing Macro (computer science) (section)

=== Applications ===
; Evaluation order
:Macro systems have a range of uses. Being able to choose the order of evaluation (see [[lazy evaluation]] and [[Strict function|non-strict functions]]) enables the creation of new syntactic constructs (e.g. [[control structures]]) indistinguishable from those built into the language. For instance, in a Lisp dialect that has <CODE>cond</CODE> but lacks <CODE>if</CODE>, it is possible to define the latter in terms of the former using macros. For example, Scheme has both [[continuation]]s and hygienic macros, which enables a programmer to design their own control abstractions, such as looping and early exit constructs, without the need to build them into the language.
; Data sub-languages and domain-specific languages
: Next, macros make it possible to define data languages that are immediately compiled into code, which means that constructs such as [[state machine]]s can be implemented in a way that is both natural and efficient.<ref>{{cite web|url=http://cs.brown.edu/~sk/Publications/Papers/Published/sk-automata-macros/|title=Automata via Macros|website=cs.brown.edu}}</ref>
; Binding constructs
:Macros can also be used to introduce new binding constructs. The most well-known example is the transformation of <CODE>let</CODE> into the application of a function to a set of arguments.

[[Matthias Felleisen|Felleisen]] conjectures<ref name="three-uses">[http://people.csail.mit.edu/gregs/ll1-discuss-archive-html/msg01539.html], Matthias Felleisen, LL1 mailing list posting</ref> that these three categories make up the primary legitimate uses of macros in such a system. Others have proposed alternative uses of macros, such as [[anaphoric macro]]s in macro systems that are unhygienic or allow selective unhygienic transformation.

The interaction of macros and other language features has been a productive area of research. For example, [[Software component|component]]s and [[Modular programming|module]]s are useful for large-scale programming, but the interaction of macros and these other constructs must be defined for their use together. Module and component-systems that can interact with macros have been proposed for Scheme and other languages with macros. For example, the [[Racket (programming language)|Racket]] language extends the notion of a macro system to a syntactic tower, where macros can be written in languages including macros, using hygiene to ensure that syntactic layers are distinct and allowing modules to export macros to other modules.