CSC/ECE 517 Fall 2011/ch1 1d gs

Introduction

This wiki gives an introduction to language constructs called closures, it’s usage and discusses about challenges involved in implementing them in statically typed languages.

Closures

A closure is a kind of routine that can be assigned to a variable or passed as a parameter to another routine. It can access the local state (local variables, parameters, methods, and instance variables) which is visible in the place it was defined.

To rephrase, a closure is a block of code which meets the following three criteria

1. It can be passed around as a value.

2. It can executed on demand by anyone who has that value, at which time.

3. It can refer to variables from the context in which it was created.

Why do we need closures and what are its uses?

DRY(Don't Repeat Yourself) is a popular software development principle, formulated by Andy Hunt and Dave Thomas, which stresses the importance of not duplicating code. Closures help in implementing the DRY principle and make the code easy to maintain.Closures increase considerably the level of a language by mixing access to local variables with remote execution of a set of locally-defined statements.

Lets see an example,

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

This function returns a closure, the behavior of which depends on the parameter amount passed to the enclosing function.

Amount can be fixed to a value like below, the variable highPaid contains a block of code (called a Proc in Ruby) that will return whether an employee’s salary is greater than 150.

 highPaid = paidMore(150)
 //Let’s check john’s salary
 john = Employee.new
 john.salary = 200
 print highPaid.call(john)

The expression highPaid.call(john) executes the e.salary > 150 block and prints the result of the execution. As long as a closure lives, all the free variables accessed by it are not eligible for garbage collection. So, the variable amount persists until the closure returned by paidMore does. Hence from the above sample code , it is clear that even if value 150 went out of scope, at the time of issuing the print call , the binding still remains.

The art of getting the best possible results by minimal code and the ease with which a user can use closures makes the latter a big success in Ruby.

A function to determine if an employee is a manager.Using C#, I'd probably write it like this.

public static IList Managers(IList emps) {
   IList result = new ArrayList();
   foreach(Employee e in emps)
     if (e.IsManager) result.Add(e);
   return result;
 }

In a language that has Closures, in this case Ruby, I'd write this.

 def managers(emps)
   return emps.select {|e| e.isManager}
 end

Statically Typed vs Dynamically Typed Languages

A programming language is said to use static typing when type checking is performed during compile-time as opposed to the run-time. Statically typed languages include Java , Objective-C ,Pascal etc.

A dynamically typed language is when the majority of its type checking is performed at run-time as opposed to that of compile-time. In dynamic typing values have types, but variables do not (a variable can refer to a value of any type) . Dynamically typed languages include Ruby,Smalltalk , Python ,JavaScript etc.

An important fact to note here is that the presence of static typing in a programming language does not necessarily imply the absence of all dynamic typing mechanisms. For example, Java and some other ostensibly statically typed languages, support downcasting and other type operations that depend on runtime type checks, a form of dynamic typing.

Closures in Dynamically Typed Languages

Closures in Ruby

Ruby is one of the languages that provides a very good support for closures. It has four different ways to define and use closures.

Scheme Closures and Scoping

Scheme is a statically scoped(link here http://web.mit.edu/scheme_v9.0.1/doc/mit-scheme-ref/Static-Scoping.html) dialect of the Lisp programming language invented by Guy Lewis Steele Jr. and Gerald Jay Sussman. It is primarily a functional programming language with very simple syntax based on s-expressions, parenthesized lists in which a prefix operator is followed by its arguments..

Scheme has same scoping as in ruby. Lets see how closures are implemented in scheme Ex #3

Defines and applies function func using free variable a

(let* ((a  ; define a

           1)
      (func                 ; define func
           (lambda () (+ a 0.1))) ; line# 4
      (func))                 ; prints 1.1

The closure on line #4 accesses the free variable a and increments its value by 0.1 Ex #4 (let* ((a 1)

      (func1   (lambda () (+ a 0.1)))
      (func2    (let* ((a   2))  func1)); Let's change a to 2, to try to make func1 use a=2
             (func2))     ; prints 1.1 from func1

func1 is called by func2, but the value printed by func1 is not influenced by the locally defined value for a.

Closures in Statically Typed Languages[A Challenge, Why?]

Java Lexical Scoping for Closures

Java does not want to permit capture of mutable local variables. The reason being that it is quite difficult to write lambda bodies that do not have race conditions. Sometimes the lambda body may be passed outside the current thread and when other threads try to execute the lambda body, race conditions may arise leaving the variables in an inconsistent state.

In short, operating with statically typed variables and assigning static types to closures’ return types, creates overhead and is cumbersome for a pure closure implementation in statically typed language.

Return from Closure

In ruby, the following function (ex2) passes a closure to function ex3. The Closure computes the value of x and returns it.

Ex #7

def ex2
   y=2
   ...
  closure=lambda { x = y + 2 }
   ...
  ex3(closure)
   ...
 end

If the same has to be achieved in C/Java, the return statement has to be used which would inturn return from the function ex2 and not execute the remaining statements within ex2.

In Ruby a closure iterates through a list of employees and returns either their name, location, skills, salary or role depending on the parameter passed to it. the return value/values of this closure change at runtime depending on the parameter passed to it.

Writing a similar closure in C/Java would require different closures for different return types.

Safety

In C, even if a nested function (platform specific, supported by GCC) is written, it cannot access the local variables in the outer function when the function exited.

Consider the following example: Ex #8

hack (int *array, int size)
    {
      void store (int index, int value)
        { array[index] = value; }
    
      intermediate (store, size);
    }

Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit. Also note, if array was a global variable and the above example is rewritten as below it would work fine.

int *array; hack( int size) {

        void store (int index, int value)
        { array[index] = value; }
    
        intermediate (store, size);
}

These are not strictly type-errors but it goes against the safety principles of a statically typed language.