How do you pass arguments to define_method?

The block that you pass to define_method can include some parameters. That's how your defined method accepts arguments. When you define a method you're really just nicknaming the block and keeping a reference to it in the class. The parameters come with the block. So:

define_method(:say_hi) { |other| puts "Hi, " + other }

... 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:

# Worksdef meth(default = :foo, *splat, &block) puts 'Bar'; end# Doesn't workdefine_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' }