Binding

Class

Objects of class Binding encapsulate the execution context at some particular place in the code and retain this context for future use. The variables, methods, value of self, and possibly an iterator block that can be accessed in this context are all retained. Binding objects can be created using Kernel#binding, and are made available to the callback of Kernel#set_trace_func and instances of TracePoint.

These binding objects can be passed as the second argument of the Kernel#eval method, establishing an environment for the evaluation.

class Demo
  def initialize(n)
    @secret = n
  end
  def get_binding
    binding
  end
end

k1 = Demo.new(99)
b1 = k1.get_binding
k2 = Demo.new(-3)
b2 = k2.get_binding

eval("@secret", b1)   #=> 99
eval("@secret", b2)   #=> -3
eval("@secret")       #=> nil

Binding objects have no class-specific methods.

Instance Methods

binding.eval(string [, filename [,lineno]]) → obj
#

Evaluates the Ruby expression(s) in string, in the binding’s context. If the optional filename and lineno parameters are present, they will be used when reporting syntax errors.

def get_binding(param)
  binding
end
b = get_binding("hello")
b.eval("param")   #=> "hello"

irb(show_code: true)
#

Opens an IRB session where binding.irb is called which allows for interactive debugging. You can call any methods or variables available in the current scope, and mutate state if you need to.

Given a Ruby file called potato.rb containing the following code:

class Potato
  def initialize
    @cooked = false
    binding.irb
    puts "Cooked potato: #{@cooked}"
  end
end

Potato.new

Running ruby potato.rb will open an IRB session where binding.irb is called, and you will see the following:

$ ruby potato.rb

From: potato.rb @ line 4 :

    1: class Potato
    2:   def initialize
    3:     @cooked = false
 => 4:     binding.irb
    5:     puts "Cooked potato: #{@cooked}"
    6:   end
    7: end
    8:
    9: Potato.new

irb(#<Potato:0x00007feea1916670>):001:0>

You can type any valid Ruby code and it will be evaluated in the current context. This allows you to debug without having to run your code repeatedly:

irb(#<Potato:0x00007feea1916670>):001:0> @cooked
=> false
irb(#<Potato:0x00007feea1916670>):002:0> self.class
=> Potato
irb(#<Potato:0x00007feea1916670>):003:0> caller.first
=> ".../2.5.1/lib/ruby/2.5.0/irb/workspace.rb:85:in `eval'"
irb(#<Potato:0x00007feea1916670>):004:0> @cooked = true
=> true

You can exit the IRB session with the exit command. Note that exiting will resume execution where binding.irb had paused it, as you can see from the output printed to standard output in this example:

irb(#<Potato:0x00007feea1916670>):005:0> exit
Cooked potato: true

See IRB Usage at IRB for more information.

binding.local_variable_defined?(symbol) → obj
#

Returns true if a local variable symbol exists.

def foo
  a = 1
  binding.local_variable_defined?(:a) #=> true
  binding.local_variable_defined?(:b) #=> false
end

This method is the short version of the following code:

binding.eval("defined?(#{symbol}) == 'local-variable'")

binding.local_variable_get(symbol) → obj
#

Returns the value of the local variable symbol.

def foo
  a = 1
  binding.local_variable_get(:a) #=> 1
  binding.local_variable_get(:b) #=> NameError
end

This method is the short version of the following code:

binding.eval("#{symbol}")

binding.local_variable_set(symbol, obj) → obj
#

Set local variable named symbol as obj.

def foo
  a = 1
  bind = binding
  bind.local_variable_set(:a, 2) # set existing local variable `a'
  bind.local_variable_set(:b, 3) # create new local variable `b'
                                 # `b' exists only in binding

  p bind.local_variable_get(:a)  #=> 2
  p bind.local_variable_get(:b)  #=> 3
  p a                            #=> 2
  p b                            #=> NameError
end

This method behaves similarly to the following code:

binding.eval("#{symbol} = #{obj}")

if obj can be dumped in Ruby code.

binding.local_variables → Array
#

Returns the names of the binding’s local variables as symbols.

def foo
  a = 1
  2.times do |n|
    binding.local_variables #=> [:a, :n]
  end
end

This method is the short version of the following code:

binding.eval("local_variables")

binding.receiver → object
#

Returns the bound receiver of the binding object.

binding.source_location → [String, Integer]
#

Returns the Ruby source filename and line number of the binding object.