Ruby lets you hook in and see (and change!) a lot of behavior of the core language. Methods, constants, classes, variables… Ruby lets you query them all, and change a lot about them.

These are just hooks – things Ruby calls in response to something happening. That’s different from, say, all the methods you can call to find our what’s defined and how – like instance variables, or method bindings, or…

Here’s summaries and links for all the hooks I could find (thanks to Google and StackOverflow!):

Methods

• respond_to_missing?: a way to make sure your dynamic methods defined with method_missing also handle respond_to? correctly
• method_missing: called when a method cannot be found, potentially to allow dynamically defining one instead
• method_added: called whenever a method is added… which can be used to modify the method
• method_removed: called whenever a method is removed
• singleton_method_added: method added to the singleton class of the object, to be callable just on this one instance
• singleton_method_removed: method removed from singleton class
• method_undefined: a method has been undefined, with undef_method. Undef_method is different from remove_method because remove_method may still allow superclasses to define the method – undef_method means it’s gone entirely.
singleton_method_undefined: called when a singleton method is undefined entirely
• initialize_copy: an optional callback when cloning any Object

Classes

• inherited: a Ruby class is subclassed

Modules

• append_features: a Module is included, and its constants, methods and variables used
• included: a Module is included, which usually obsoletes “append_features”
• extend_object: a Module extends an Object
• extended: an Object is extended by a module, which mostly obsoletes extend_object
• const_missing: a constant isn’t already present

Marshalling

• marshal_dump: called on an object to have it dump itself in Ruby Marshal format
• marshal_load: called on an object to have it load itself in Ruby Mashal format

Coercion

• coerce: called by the first type in a two-argument operator on the second argument, to make it turn into something the first argument can recognize
• induced_from: deprecated, please don’t use
• to_i, to_f, to_s, to_a, to_hash, to_proc and others: conversions, indicating that the object is being used as a type and should try to convert itself to that type