CSC/ECE 517 Fall 2007/wiki1 2 pk

Callbacks

A callback is executable code that is passed as an argument to other code. It allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer. A callback can be used as a simpler alternative to polymorphism and generic programming, in that the exact behavior of a function can be dynamically determined by passing different (yet compatible) function pointers or handles to the lower-level function. This can be a very powerful technique for code reuse.

Normally a callback uses such a parameter as a pointer to application data outside its own scope. This feature is necessary only in a statically scoped language like C or C++ . Dynamically scoped languages (like Ruby and Small Talk) can provide access to application data automatically via closures.

Closures are functions which are evaluated in an environment containing bound variables. They are blocks of code which meets three criteria:

• They can be passed around as a value and
• They can be executed on demand
• They can refer to variables from the context in which it was created

Closures were initally developed in the 1960s in a language called Scheme and later adopted in other languages including Ruby and Smalltalk.

Examples

The code below illustrates a simple case of closure use.

def thrice
    yield #Pass control back to block
    yield
    yield
end


x = 5
puts "value of x before: #{x}"
thrice { x += 1 }
puts "value of x after: #{x}"

The closure(or block) { x+=1 } is passed as a parameter to the function 'thrice'. The value of x before 'thrice' is called is 5. On execution the value becomes 8 i.e. the block {x+=1} was executed thrice.

However, the biggest advantage of closures is that they can refer to the variables which were visible in the lexical environment of the function in which they were defined.

def highPaid(emps)
  threshold = 150
  return emps.select {|e| e.salary > threshold}
end

Closures can also refer to the arguments of the function in which it is defined. For example, conside the ruby code snippet below.

def paidMore(amount)
  return Proc.new {|e| e.salary > amount}
end

john = Employee.new
john.salary = 200

highPaid = paidMore(150)
print highPaid.call(john)