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 ...
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