Ruby Method Object, define_method and bind

Ruby methods are executable language constructs. They are not objects. But we can get objects that represents methods with the Object class's method method.

Note: This is similar to how blocks are language syntax but not objects in Ruby and we can get object versions of blocks in the forms of procs and lambdas.

So the Object#method takes a method name - either string or symbol - and return an Method object. This object represents the named method on the receiver.

l = "hello".method(:length)
puts #will return 5

Here we get a Method representing the length method on the String call, bound to the string "hello". And we can call this method with Method#call.

Note: We can use public_method which works the same as method but ignores protected and private methods.

Method to Proc and Proc to Method

Method objects work very much like Proc objects and can be used in place of them. Method even has a to_proc method to convert it to Proc if necessary. Here is an example where the method is passed in place of a proc with an &:

def square(x)
  x * x
puts (1..10).map(&method(:square))

We can also go in the other direction. Instead of getting Method objects and converting them to Proc, we can create a Method from blocks and procs using define_method of the Module class.

define_method expects a Symbol as an argument and create a method with that name using the associated block as the method body like this define_method(:say_hello) { puts "hello" }. This is the same as if we had done this:

def say_hello
  puts "hello"

Unbounded Method objects and bind

So in the examples above we get Method objects that are tied to an instance of a class that contains the method. These Method objects have a receiver. Can we get a Method object that was not bounded to an instance?

Turns out we can. It would be of a class called UnboundMethod though. And we cannot actually call this UnboundMethod as it wouldn't make sense. For example

unbounded_power = Fixnum.instance_method("**")

creates a unbounded_power method that is of type UnboundMethod. And in order to actually invoke this method we can have to bind it to something

power_of_2 = unbounded_power.bind(2)
puts # prints 32

The power_of_2 is a Method object now and we can use its call method.

Lastly, Method has some other handy methods name, owner, and receiver that work as below.

=> :**

> power_of_2.owner
=> Integer

> power_of_2.receiver
=> 2

That's all about Method objects in Ruby and how they compare to Procs.

Show Comments