How do you pass arguments to define_method?

... and if you want optional parameters

 class Bar define_method(:foo) do |arg=nil| arg end end a = Bar.new a.foo #=> nil a.foo 1 # => 1 

... as many arguments as you want

 class Bar define_method(:foo) do |*arg| arg end end a = Bar.new a.foo #=> [] a.foo 1 # => [1] a.foo 1, 2 , 'AAA' # => [1, 2, 'AAA'] 

...combination of

 class Bar define_method(:foo) do |bubla,*arg| p bubla p arg end end a = Bar.new a.foo #=> wrong number of arguments (0 for 1) a.foo 1 # 1 # [] a.foo 1, 2 ,3 ,4 # 1 # [2,3,4] 

... all of them

 class Bar define_method(:foo) do |variable1, variable2,*arg, &block| p variable1 p variable2 p arg p block.inspect end end a = Bar.new a.foo :one, 'two', :three, 4, 5 do 'six' end 

Update

Ruby 2.0 introduced double splat ** (two stars) which (I quote) does:

Ruby 2.0 introduced keyword arguments, and ** acts like *, but for keyword arguments. It returns a Hash with key / value pairs.

...and of course you can use it in define method too :)

 class Bar define_method(:foo) do |variable1, variable2,*arg,**options, &block| p variable1 p variable2 p arg p options p block.inspect end end a = Bar.new a.foo :one, 'two', :three, 4, 5, ruby: 'is awesome', foo: :bar do 'six' end # :one # "two" # [:three, 4, 5] # {:ruby=>"is awesome", :foo=>:bar} 

Named attributes example:

 class Bar define_method(:foo) do |variable1, color: 'blue', **other_options, &block| p variable1 p color p other_options p block.inspect end end a = Bar.new a.foo :one, color: 'red', ruby: 'is awesome', foo: :bar do 'six' end # :one # "red" # {:ruby=>"is awesome", :foo=>:bar} 

I was trying to create example with keyword argument, splat and double splat all in one:

 define_method(:foo) do |variable1, variable2,*arg, i_will_not: 'work', **options, &block| # ... 

or

 define_method(:foo) do |variable1, variable2, i_will_not: 'work', *arg, **options, &block| # ... 

... but this will not work, it looks like there is a limitation. When you think about it makes sense as splat operator is "capturing all remaining arguments" and double splat is "capturing all remaining keyword arguments" therefore mixing them would break expected logic. (I don't have any reference to prove this point doh! )

update 2018 August:

Summary article: https://blog.eq8.eu/til/metaprogramming-ruby-examples.html

In addition to Kevin Conner's answer: block arguments do not support the same semantics as method arguments. You cannot define default arguments or block arguments.

This is only fixed in Ruby 1.9 with the new alternative "stabby lambda" syntax which supports full method argument semantics.

Example:

# Works def meth(default = :foo, *splat, &block) puts 'Bar'; end # Doesn't work define_method :meth { |default = :foo, *splat, &block| puts 'Bar' } # This works in Ruby 1.9 (modulo typos, I don't actually have it installed) define_method :meth, ->(default = :foo, *splat, &block) { puts 'Bar' } 

With 2.2 you can now use keyword arguments: https://robots.thoughtbot.com/ruby-2-keyword-arguments

define_method(:method) do |refresh: false| .......... end