CSC/ECE 517 Fall 2011/ch3 4b js

Closures

What is a Closure?

A closure is a block of code that can access the lexical environment of its definition. A closure has two properties:

• A closure can be passed as an object.
• A closure recalls the values of all the variables that were in scope when the function was created and is able to access those variables when it is called even though they may no longer be in scope.

A closure can be expressed simply as a function pointer that references a block of executable code and the variables from the scope it was created.

Closures in Ruby

Ruby supports closures through use of Procedures, or simply procs, and lambdas, which are Blocks. When creating a lambda or proc, the object holds a reference to the executable block and bindings for all the variables used by the block.

Procs and Lambdas

In Ruby, we can create a Proc object explicitly in three different ways.

• Using Proc.new – Using this method involves simply passing in a block, which will return a Proc object that will run the code in the block when you invoke its call method.
  • pobject = Proc.new {puts “This is a proc object.”}
  • pobject.call
• Using the proc method - In the Kernel module, we can use the proc method, which is available globally. In Ruby 1.9, it is equivalent to Proc.new, but in Ruby 1.8, it is equivalent to lambda. The difference between proc and lambda will be discussed later.
  • pobject = proc { puts “Inside the proc object” }
  • pobject.call
• Using the lambda method in the Kernel module – Similar to the proc method, it is globally available in the Kernel module, however, it will create a lambda Proc object.
  • pobject = proc { puts “Inside the proc object” }
  • pobject.call

The difference between Procs and Lambdas has to do with their handling of control flow keywords, such as break and return. http://www.skorks.com/2010/05/ruby-procs-and-lambdas-and-the-difference-between-them/#difference