Exactly what is the difference between a “closure” and a “block”?

Question

I\'ve found that lots of people use the words closure and block interchangeably. Most of these people can\'t explain what they\'re talking about.

S

Answer 1

A block is something syntactical - A logical unit of statements (more related to scope than to closure).

if (Condition) {
    // Block here 
} 
else {
    // Another block
}

A closure is related to anoymous functions or classes - An anonymous (function) object, a piece of code that is bound to an environment (with its variables).

def foo() {
   var x = 0
   return () => { x += 1; return x }
}

Here foo returns a closure! The local variable x persists through the closure even after foo terminated and can be incremented through calls of the returned anonymous function.

val counter = foo()
print counter() // Returns 2
print counter() // Return 3

Note that's just Ruby in which block and closure are treated similarly since what Ruby calls block is a closure:

(1..10).each do |x|
    p x
end

There each-method is passed a closure function (taking a parameter x) which is called a block in Ruby.