CSC/ECE 517 Fall 2012/ch1 1w4 aj

Introduction

Code Blocks

Before understanding the concept of Closures, let us take a brief introduction on “Code Blocks” in Ruby. A code block is a chunk of code, Ruby statements and expressions written between curly braces { } or between “do…end”. For example:

 {  puts "Hello World!"  }

or

  do
     3.times(puts "Hello")
     object1.call
  end

Generally, as per Ruby standard, braces are used for single-line blocks and do...end for multiline blocks. Also, braces have a higher preference than do/end. A code block may appear only immediately after a method is invoked. If a method has parameters, then the block will look as follows:

 random_method("John") { puts "How you doing? " }

Closures in Ruby

What is a Closure?

Now, a block as shown before can use local variables from the surrounding scope. Such blocks are called Closures. Let us look at a simple example:

def closurefunc()
     lambda {|val| val + inc }
  end

  p1 = closurefunc(3)
  p1.call(1)    # => 4
 
  p2 = closurefunc(8)
  p2.call(5)   # => 13

In the above example, the value ‘3’ is assigned to the local variable inc of method closurefunc and value ‘1’ is assigned to inner variable val.

How does it work?

Let us look at a similar example, but this time, with strings.

 
def concat()
     lambda { |greet| greet + param }
  end

  p1 = concat("John")
  p1.call("Hello")    # => "HelloJohn"
 
  p2 = concat("Jim")
  p2.call("Good morning ")   # => "Good morning Jim"

The method concat returns a Proc object that references the method’s parameter, param. We need to use the call method of the Proc object to execute it. Even though the parameter param is out of scope when the block is called, the parameter is still accessible to the block. This is called a closure; where in the variables in the surrounding scope referenced in a block remain accessible for the life of that block and the life of any Proc object created from that block.




Why Closure is needed?

Other ways to use a closure

Rather than calling the method lambda, following ways can be used to create Proc objects:
Using the ( -> ) syntax
->params { . . . }
Example:
obj = ->(v1,v2) { v1 + v2 }
obj.call(2,3) # => 5
Note: There cannot be any space between > and opening paranthesis.

By passing a block to the method whose last parameter is prefixed with and ampersand That parameter will receive the block as a Proc object.

def sampmeth(v1, &block)
    puts block.inspect
 end
  sampmeth(1) { "a block" }
  sampmeth(3)

produces:

   #<Proc:0x0b5f5e@/tmp/prog1.rb:4>
  Nil

By calling Proc.new by associating it with a block
obj = Proc.new { "a block" }
obj
=> #<Proc:0x0a5e5a@/tmp/prog1.rb:1>