How to do method binding (or re-binding) in Ruby

…or how to decorate object so that it can be undecorated:

class Account
  attr_reader :state

  def initialize(state)
    @state = state
  end
end

module Debit
  def transaction(amount)
    @state = @state - amount
  end
end

module Credit
  def transaction(amount)
    @state = @state + amount
  end
end

account = Account.new(100)
account.state                       # => 100
puts account.public_methods(false)  # => [:state]


debit = Debit.instance_method(:transaction)
credit = Credit.instance_method(:transaction)


# Lets do debit transactions
transaction = debit.bind(account)
transaction.call(3)
transaction.call(3)

account.state                       # => 94
puts account.public_methods(false)  # => [:state]
puts account.public_methods(false)  # => [:state]


# Lets do credit transactions
transaction = credit.bind(account)
transaction.call(7)
transaction.call(7)

puts account.state                  # => 108
puts account.public_methods(false)  # => [:state]

Note! this is still experimental feature in Ruby and too fast.

So when you think about it, we manage to decorate the object with a method, called some logic that changed the state and then we were able to work with the original object (without that method)

Note! This is still quite slow feature in Ruby. I personally wouldn’t use it in a production code. But it’s cool that something like this exist.

Unlike include, extend, inheritance this is reversable => you can work with original object without that logic. With include/extend once you go that way you cannot come back:

module Foo
  def foo
    123
  end
end

a = Object.new
b = a
a.extend(Foo)
a.foo   # => 123
b.foo   # => 123

Well yeah you could, like with Refinements but that’s not my point. You should be able to access the original (undecorated) object.

And although you can do similar transformations with decorator objects (e.g. like SimpleDecorator) the point is that you have to define class where you pass object in order to do transformation. You are not directly injecting / removing functions from object (bind/unbind).

Sources

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