How do you pass arguments to define_method? - ruby

Question

I would like to pass an argument(s) to a method being defined using define_method, how would I do that?

Answer 1

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 }

Answer 2

... 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

Answer 3

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' }

Answer 4

With 2.2 you can now use keyword arguments:
https://robots.thoughtbot.com/ruby-2-keyword-arguments
define_method(:method) do |refresh: false|
..........
end