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A macro is a piece of code that executes at compile time and either performs textual manipulation of code to-be compiled (e.g. C++ macros) or manipulates the abstract syntax tree being produced by the compiler (e.g. Rust or Lisp macros). Textual macros are notably more independent of the syntax of the language being manipulated, as they merely ...
However, syntax-case is more powerful than syntax-rules. For example, syntax-case macros can specify side-conditions on its pattern matching rules via arbitrary Scheme functions. Alternatively, a macro writer can choose not to use the pattern matching frontend and manipulate the syntax directly.
The first application run in such a context is a more sophisticated and powerful macro compiler, written in the machine-independent macro language. This macro compiler is applied to itself, in a bootstrap fashion, to produce a compiled and much more efficient version of itself. The advantage of this approach is that complex applications can be ...
Macro languages may be restricted to acting on specially labeled code regions (pre-fixed with a # in the case of the C preprocessor). Alternatively, they may not, but in this case it is still often undesirable to (for instance) expand a macro embedded in a string literal, so they still need a rudimentary awareness of syntax. That being the case ...
Metaprogramming enables developers to write programs and develop code that falls under the generic programming paradigm. Having the programming language itself as a first-class data type (as in Lisp , Prolog , SNOBOL , or Rebol ) is also very useful; this is known as homoiconicity .
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Procedural macros are Rust functions that run and modify the compiler's input token stream, before any other components are compiled. They are generally more flexible than declarative macros, but are more difficult to maintain due to their complexity. [125] [126] Procedural macros come in three flavors:
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