Metaprogramming in Ruby Created By Kristen Mills 

What is Metaprogramming? 

The Basics 

Symbols Kind of like strings Immutable Not garbage collected 

Blocks That do...end thing or curly braces if on one line One of the 3 types of closures 

Monkey Patching 

Monkey Patching by example c l a s s S t r i n g d e f m y _ m e t h o d " m y _ m e t h o d i s a c o o l m e t h o d " e n d e n d ' a b c ' . m y _ m e t h o d # = > " m y _ m e t h o d i s a c o o l m e t h o d " 

Refinements Allows you to monkey patch in a given scope 

Aliasing method alias method_1, method_2 alias_method :method_1, :method_2 

Defining Methods def method_name(params) define_method(name, &block) define_singleton_method(name, &block) 

Instance Variables instance_variable_set(name, value) instance_variable_get(name) 

attr_accessor d e f a t t r _ a c c e s s o r ( * a r g s ) a r g s . e a c h d o | a r g | d e f i n e _ m e t h o d ( a r g ) d o i n s t a n c e _ v a r i a b l e _ g e t ( : " @ # { a r g } " ) e n d d e f i n e _ m e t h o d ( : " # { a r g } = " ) d o | v a l u e | i n s t a n c e _ v a r i a b l e _ s e t ( : " @ # { a r g } " , v a l u e ) e n d e n d e n d 

Eigenclasses! 

Wait, where does that method go? 

Accessing the Eigenclass e i g e n c l a s s = c l a s s < < o b j c l a s s < < a n _ o b j e c t # y o u r c o d e h e r e e n d 

Before you ask Yes, eigenclasses have Eigenclasses 

Calling Methods send(name, *args) 

Removing Methods remove_method(name) undef_method(name) 

Rails Black Magic (aka Method Missing) 

Method Missing method_missing(method, *args, &block) 

Domain Specific Languages 

Evaluating Strings/Blocks eval(string) instance_eval(string) instance_eval(&block) instance_exec(*args, &block) yield(*args) call(*args) 

Vending Machine f i n i t e i n i t i a l : : i d l e d o b e f o r e : i d l e d o @ c u r r e n t _ m o n e y = 0 e n d a f t e r : a c c e p t i n g d o p u t s " C u r r e n t a m o u n t i n m a c h i n e : $ % . 2 f " % @ c u r r e n t _ m o n e y e n d @ m o n e y . e a c h d o | e v e n t _ n a m e , a m o u n t | e v e n t : " i n s e r t _ # { e v e n t _ n a m e } " d o b e f o r e d o p u t s " A d d i n g # { e v e n t _ n a m e } " a d d _ m o n e y ( a m o u n t ) e n d g o f r o m : : i d l e , t o : : a c c e p t i n g g o f r o m : : a c c e p t i n g , t o : : a c c e p t i n g e n d e n d @ p r o d u c t s . e a c h d o | e v e n t _ n a m e , p r i c e | e v e n t : " b u y _ # { e v e n t _ n a m e } " d o b e f o r e { p u t s " B u y i n g # { e v e n t _ n a m e } " } g o f r o m : : a c c e p t i n g , t o : : v e n d i n g , i f : l a m b d a { @ c u r r e n t _ m o n e y > = p r i c e } a f t e r d o @ c u r r e n t _ m o n e y - = p r i c e e n d e n d e n d e v e n t : c o m p l e t e _ v e n d d o b e f o r e { p u t s " R e t u r n i n g $ % . 2 f t o t h e c u s t o m e r " % @ c u r r e n t _ m o n e y } g o f r o m : : v e n d i n g , t o : : i d l e e n d e v e n t : p r e s s _ c o i n _ r e t u r n d o 

add_event d e f a d d _ e v e n t ( e v e n t _ n a m e , & b l o c k ) # S o m e o t h e r s t u f f h a p p e n s b e f o r e h e r e @ c l a s s . s e n d ( : d e f i n e _ m e t h o d , : " c a n _ # { e v e n t _ n a m e } ? " ) d o e v e n t . t r a n s i t i o n s . k e y ? c u r r e n t _ s t a t e . n a m e e n d @ c l a s s . s e n d ( : d e f i n e _ m e t h o d , : " # { e v e n t _ n a m e } " ) d o i f e v e n t . t r a n s i t i o n s . k e y ? c u r r e n t _ s t a t e . n a m e t r a n s i t i o n = e v e n t . t r a n s i t i o n s [ c u r r e n t _ s t a t e . n a m e ] u n l e s s t r a n s i t i o n . c o n d i t i o n . n i l ? o r s e l f . i n s t a n c e _ e x e c ( & t r a n s i t i o n . c o n d i t i o n ) r a i s e E r r o r . n e w ( ' D o e s n o t m e e t t h e t r a n s i t i o n c o n d i t i o n ' ) e n d n e w _ s t a t e = s t a t e s [ e v e n t . t r a n s i t i o n s [ c u r r e n t _ s t a t e . n a m e ] . t o ] e v e n t . c a l l b a c k s [ : b e f o r e ] . e a c h d o | c a l l b a c k | s e l f . i n s t a n c e _ e v a l & c a l l b a c k e n d # M o r e c a l l b a c k s h a p p e n h e r e @ c u r r e n t _ s t a t e = n e w _ s t a t e # M o r e C a l l b a c k s h a p p e h e r e e v e n t . c a l l b a c k s [ : a f t e r ] . e a c h d o | c a l l b a c k | s e l f . i n s t a n c e _ e v a l & c a l l b a c k e n d s e l f e l s e r a i s e E r r o r . n e w ' I n v a l i d T r a n s i t i o n ' e n d e n d e n d 

Any Questions? 

Thanks!