Ruby metaprogramming with singleton_class

ruby-metaprogramming-singleton_class.rb2

[1] pry(main)> a = Object.new
=> #<Object:0x007f90fc8f40f0>
[2] pry(main)> a.singleton_class
=> #<Class:#<Object:0x007f90fc8f40f0>>
[3] pry(main)> a.singleton_class.send(:define_method, "asdf") { |a| "hi there " + a }
=> :asdf
[4] pry(main)> a.asdf
ArgumentError: wrong number of arguments (0 for 1)
from (pry):3:in `block in __pry__'
[5] pry(main)> a.asdf("gg")
=> "hi there gg"
[6] pry(main)> b = Object.new
=> #<Object:0x007f90fa981bc8>
[7] pry(main)> b.asdf("gg")
NoMethodError: undefined method `asdf' for #<Object:0x007f90fa981bc8>
from (pry):7:in `__pry__'

[1] pry(main)> a = "Hello"
=> "Hello"
[2] pry(main)> a.class.send(:define_method, "asdf") { "Hi there!" }
=> :asdf
[3] pry(main)> a.asdf
=> "Hi there!"
[4] pry(main)> b = "Hello again!"
=> "Hello again!"
[5] pry(main)> b.asdf
=> "Hi there!"
[6] pry(main)> a.singleton_class.send(:define_method, "fdsa") { "Hey there again!" }
=> :fdsa
[7] pry(main)> a.fdsa
=> "Hey there again!"
[8] pry(main)> b.fdsa
NoMethodError: undefined method `fdsa' for "Hello again!":String
from (pry):8:in `__pry__'

[9] pry(main)> a.singleton_class.send(:define_method, "aaaa") { |a, b| "And the result is: " + a.to_s + b.to_s }
=> :aaaa
[10] pry(main)> a.aaaa(1, 2)
=> "And the result is: 12"
[11] pry(main)> a.respond_to?(:aaaa)
=> true
[12] pry(main)> b.respond_to?(:aaaa)
=> false
[13] pry(main)> c = a.singleton_class.new
TypeError: can't create instance of singleton class
from (pry):13:in `new'
[14] pry(main)> Object.new.methods
=> [:pry,
 :__binding__,
 :pretty_print,
 :pretty_print_cycle,
 :pretty_print_instance_variables,
 :pretty_print_inspect,
 :nil?,
 :===,
 :=~,
 :!~,
 :eql?,
 :hash,
 :<=>,
 :class,
 :singleton_class,
 :clone,
 :dup,
 :itself,
 :taint,
 :tainted?,
 :untaint,
 :untrust,
 :untrusted?,
 :trust,
 :freeze,
 :frozen?,
 :to_s,
 :inspect,
 :methods,
 :singleton_methods,
 :protected_methods,
 :private_methods,
 :public_methods,
 :instance_variables,
 :instance_variable_get,
 :instance_variable_set,
 :instance_variable_defined?,
 :remove_instance_variable,
 :instance_of?,
 :kind_of?,
 :is_a?,
 :tap,
 :send,
 :public_send,
 :respond_to?,
 :extend,
 :display,
 :method,
 :public_method,
 :singleton_method,
 :define_singleton_method,
 :object_id,
 :to_enum,
 :enum_for,
 :gem,
 :pretty_inspect,
 :==,
 :equal?,
 :!,
 :!=,
 :instance_eval,
 :instance_exec,
 :__send__,
 :__id__]
[15] pry(main)> a.singleton_methods
=> [:fdsa, :aaaa]