The Ruby Programming Language

Chapter 8. Reflection and Metaprogramming

We’ve seen that Ruby is a very dynamic language; you can insert new methods into classes at runtime, create aliases for existing methods, and even define methods on individual objects. In addition, it has a rich API for reflection. Reflection, also called introspection, simply means that a program can examine its state and its structure. A Ruby program can, for example, obtain the list of methods defined by the Hash class, query the value of a named instance variable within a specified object, or iterate through all Regexp objects currently defined by the interpreter. The reflection API actually goes further and allows a program to alter its state and structure. A Ruby program can dynamically set named variables, invoke named methods, and even define new classes and new methods.

Ruby’s reflection API—along with its generally dynamic nature, its blocks-and-iterators control structures, and its parentheses-optional syntax—makes it an ideal language for metaprogramming. Loosely defined, metaprogramming is writing programs (or frameworks) that help you write programs. To put it another way, metaprogramming is a set of techniques for extending Ruby’s syntax in ways that make programming easier. Metaprogramming is closely tied to the idea of writing domain-specific languages, or DSLs. DSLs in Ruby typically use method invocations and blocks as if ...