Java8:Deep dive to Lambda as a Closure

In Java8 the most important inclusion is the lambda, It is important because  using lambda we can manipulate function/method like data member , also we can pass or returns a function from another function(using java Functional interface). Prior to java8, we can do it by anonymous class but it has a very complex signature.

Now the questions are
what is closure?
How does lambda serve as a closure?
java 8 follows pure closure?
How is Closure useful?

We are going to discuss it one by one.

what is closure?

In a very simple term closure is a function/behavior which uses a variable which is defined out of the scope of this function(method in Java).

This is very common case,

Ex:

public class Greet{
String greet =”Hello”;
public String doGreet(String name){
return greet + name;
}

}

Here, greet is a variable which declared outside of the function scope and we use it in doGreet() function. But the problem arises when a function uses or manipulate an external variable inside a closing function and returns the closing function as a return value.
Hard to understand, let think in this way,  in Java 8 we can pass or returns a function by help of the functional interface, so we can say,  like the value we can also return a function which uses an external variable.

Let's modify the code a bit using Java 8 lambda

package com.example.closure;

import java.util.function.Supplier;

public class Greet {
   
   public Supplier<String>doGreet(String name){
      String salutation = "Hello";
      return () ->salutation + " "+ name;    
             
   }
   
   public static void main(String[] args) {
      Greet greet = new Greet();
      Supplier<String> supplier = greet.doGreet("Shamik");
      String message = supplier.get();
      System.out.println(message);
     
   }

}

How does lambda serve as a closure?

Here, I just modify the code little bit, previously doGreet() directly returns a value by concatenating the name and salutation, Here I return a Functional Interface from doGreet() called Supplier(Java8 built-in functional interface) which takes nothing but produces a String, By doing this I can use that intermediate function, later on.

Here Salutation variable is declared inside doGreet() method but salutation variable used in the intermediate function Supplier<String>, so the state of the salutation variable bound into the intermediate function which is used later when we call supplier.get(). So the state of a variable
Which is defined outside the function is closed by the function or encapsulated by the function, hence the name is closure, Close the state of a foreign variable inside a function.

The closure is a function which wraps a state of a variable which is defined outside of the function and can manipulate the state.


java 8 follows pure closure?

In our example Supplier, the functional interface acts as a closure as it encapsulates the salutation variable which is defined outside of the intermediate function.

But as per the closure definition, we can manipulate the state of the external variable so let's try to modify the salutation variable inside the function.

package com.example.closure;

import java.util.function.Supplier;

public class Greet {
   
   public Supplier<String>doGreet(String name){
      String salutation = "Hello";
      return () ->
      {
          salutation = "welcome";//modifying the state
          return salutation + " "+ name;    
      };
             
   }
   
   public static void main(String[] args) {
      Greet greet = new Greet();
      Supplier<String> supplier = greet.doGreet("Shamik");
      String message = supplier.get();
      System.out.println(message);
     
   }

}


If we try to compile the above code we got a compilation error, the error says
“Local variable salutation defined in an enclosing scope must be final or effectively final”

What it tries to say is -- as salutation defined in doGreet() method, so the scope  of salutation live as long as the doGreet() method executes  by main thread, as the variable creates in stack memory. But here we use  the salutation variable inside the intermediate function Supplier<String>, we can only do this in Java if and only if we guarantee that the value of the salutation is never changed , so by the final modifier we can provide this guarantee  in java8 whether you declare final explicitly or not if you use the variable which is defined outside and use the same in lambda , java8 treats that variable as effective final, whose reference never changed.


But here we try to modify the value inside the intermediate function by assigning the value “welcome”  so the compiler complains the above.


But as per closure definition, we can manipulate the same so Java8 does not act as a pure closure but we can use the value defined outside of the function scope without modifying it which is also very useful, although Java lambda is not a pure closure but it serves most of the closure functionality very well.




How Closure is useful ?

As closure encapsulate a state of variable, so we can apply the state elsewhere in the code which brings a great dynamicity into the code, By closure, we can wrap a behavior and local state together and apply the same as a callback, so we create our own strategy and inject the same into the framework. It is the implementation of Strategy pattern in Functional programming.


Conclusion :Although in Java we have Closure in form of anonymous class previously, java 8 add some syntactic sugar to it. But in spite of that, lambda is very easy to use and we can implement some of the functional style library function, curry function. Which is very tedious to implement using an anonymous class.



Function Curry in Java8




One of the important features of a functional program is Function Currying.As we know java8 introduce lambda which brings some functional nature in java programming. We are blessed that by an intelligent use of Lambda we can create Function currying by own. Although Java8 does not have Currying facilities we can create it by ourselves. In this section, I will discuss the detail step How to create Currying in Java 8.



What is a Function Currying?
To understand the function currying let first understand what is a Partial function.

A partial function is the application of a part of a function.

To be very simple, Suppose I have a java method called add which takes two arguments as input then capability of applying one argument is called the Partial function.

Maybe you are astonished in this moment why the hell we need to do that?

Let me tell you, This is a very rich concept and it helps you to write a rich API and reduce the Repeating of yourself (DRY). It actually eliminates the conceptual duplication.

Now I am trying to describe it with a example so you can understand the above paragraph well.

Suppose, I want to write a generic compute function which takes two inputs and takes a BiFunction (Java 8 functional Interface) so I can pass behavior on the fly which will be applied to the two inputs argument.

Example

public  Integer compute(Integer a, Integer b,BiFunction<Integer,Integer,Integer> function){      
       return function.apply(a,b);
    }


By this compute method I can apply add, multiply, division whatever on these passing parameters a and b.

So I will call the Compute function in following way
BiFunction<Integer,Integer,Integer> addFunction =  (a,b)-> a+b;
Integer summation = fc.compute(50, 20, addFunction);
System.out.println("Summation is " + summation);

The output will be 70, as BiFunction called addFunction which do the adding.

Now suppose we have to build another logic, Say I pass a number then a base will automatically apply to it. The base value is  10.

So if the base is 10 and I pass 20 then the output will be 10 + 20 =30(base + actual parameter).

How would you write this function?

Very easy isn’t it, we should write it in the following manner

public Integer baseCompute(Integer value,Function<Integer,Integer> function){
       return function.apply(value);
    }


It takes a value and a Function which takes one Integer as input and return another Integer So that this function takes the passing value apply base to it and return the resultant value.

We call this baseCompute method by the following call,

Function<Integer,Integer> baseFunction =  (value)-> 10 + value;//declare base
      Integer baseResult = fc.baseCompute(20,baseFunction);
      System.out.println("Base computation " + baseResult);


The output will be 30.
So far so good, But if you are a Clean coder you are not happy with it because you observe

BiFunction<Integer,Integer,Integer> addFunction =  (a,b)-> a+b;
Function<Integer,Integer> baseFunction =  (value)-> 10 + value;

The addFunction and baseFunction body are same, so you duplicate your code breaking the DRY principle. So you must search for an option where you can reuse the same addFunction instead of writing a new baseFunction, But the problem is the Structure of addFunction is different from the base function. addFunction takes two parameters and base takes one, But conceptually both are doing same work,

addFunction add two parameters, baseFunction takes one parameter and add base value internally.

Think How you can solve the problem while I taking a short break for breakfast.

We can solve this problem, by fixing a parameter in the bi-function. As we know the base value is 10 so if we apply that value and take another value from caller then we can reuse the BiFunction.

So we could rewrite the baseCompute method like following

 public Integer applyPartialInternalCompute(Integer value,BiFunction<Integer,Integer,Integer> function){
       return function.apply(10,value);
    }


And calls the above method like following

BiFunction<Integer,Integer,Integer> addFunction =  (a,b)-> a+b;
Integer baseInternal = fc.applyPartialInternalCompute(20, addFunction);
 System.out.println("Base Internal " + baseInternal);



Congrats you have written your first partial function, Where you fixed you first parameter and takes a parameter from Caller.

It is the essence of Partial function apply a partial part of the function and returns a new function which can take remaining arguments. So it is consists of two thing



  1. Applying a part.
  2. Return a new function after applying one argument.

Mathametically

f ( x , y ) = x+y
After applying the  first parameter of function
f(2,y)=N(y)=2+y; where N(Y) is a new function returned after currying.


But what we did in our applyPartialInternalCompute method is not returning a new function or the intermediate function after partial applying the first value- Which is base value,  so it is not properly curryed this is very basic version, Let see how can we improve the function so we can get the new intermediate function after applying the first argument base.

public Function<Integer,Integer> applyPartialExternalCompute(Integer base,BiFunction<Integer,Integer,Integer> function){
       return value->function.apply(base,value);
    }


This is a more sophisticated version, It takes a Bi-function and a base value, Then apply the base value to the BiFunction and return a new Function which takes One parameter from the caller and returns another Integer.

So this function works as an adapter, It takes Bi-function applying a parameter or fixing a parameter and returns new Intermediate function to work upon,


BiFunction<Integer,Integer,Integer> = Fixed Base value + Function<Integer,Integer>

Please note that the returning function is looks  like of our baseFunction   Function<Integer,Integer> baseFunction =  (value)-> 10 + value;//declare base

This is called Function Currying, so we can use the BiFunction at fullest, So a BiFunction can take two parameters also it serves any functionality which takes one argument by fixing the first one.

How to call the applyPartialExternalCompute method,

Integer basepartial= fc.applyPartialExternalCompute(10, addFunction).apply(20);
System.out.println("Base External " + basepartial);


Our above solution is very good, and we properly use function Currying to remove conceptual duplication but wait there is still some glitch in the method we have written.

It takes Bi-function and a value and returns a Function, So every time we invoke this method it applies the parameter to BiFunction and return a new Function, But why should we perform two steps every time if we got the readymade intermediate function then we just apply our values,
Suppose there is a function which takes three arguments then we have to write two adapter functions one takes tri-function and return a Bi-function and another takes Bi-function and return a function,

So if parameter values are n we have to write n-1 adaptor methods. Which is again a duplication, So if the Curry function itself generates all intermediate structures then it will be a full proof solution lets implement this,

public   Function<Integer,Function<Integer,Integer>> applyCurryingCompute(BiFunction<Integer,Integer,Integer> function){
       return value->(base->function.apply(base,value));
    }


Yes, we are waiting for this implementation, and it is the Crux of this article, By above Curry function we should have every intermediate function and we can do any functionality by fixing others,

It takes a Bi-Function which returns an Intermediate function which takes an argument and returns a function which again takes another argument and lastly returns an Integer.

Hard to understand,

Then see the following transformation to understand what actually going on

(x,y)->z = x->(y->z)   BiFunction changes it structure after currying and create a new structure
(Integer,Integer)=Integer - > Integer =Integer->( Integer->Integer)

f(y)=x->f(x,y)=z where x->f(x,y) means x applied on Bifunction creates a function of y which is equivalent to Z

So (x->y)->z = x->(y->z).


How to call above applyCurryingCompute method

Integer baseCurry= fc.applyCurryingCompute(addFunction).apply(10).apply(20);
System.out.println("Base Curry " + baseCurry);


The full version of the program

package com.example.function.curry;

import java.util.function.BiFunction;
import java.util.function.Function;

public class FunctionCurrying {
   
    public  Integer compute(Integer a, Integer b,BiFunction<Integer,Integer,Integer> function){      
       return function.apply(a,b);
    }
    public Integer baseCompute(Integer value,Function<Integer,Integer> function){
       return function.apply(value);
    }
   
    public Integer applyPartialInternalCompute(Integer value,BiFunction<Integer,Integer,Integer> function){
       return function.apply(10,value);
    }
    public Function<Integer,Integer> applyPartialExternalCompute(Integer base,BiFunction<Integer,Integer,Integer> function){
       return value->function.apply(base,value);
    }
   
    public   Function<Integer,Function<Integer,Integer>> applyCurryingCompute(BiFunction<Integer,Integer,Integer> function){
       return value->(base->function.apply(base,value));
    }
   
   
    public static void main(String[] args) {
       FunctionCurrying fc = new FunctionCurrying();
       BiFunction<Integer,Integer,Integer> addFunction =  (a,b)-> a+b;
       Function<Integer,Integer> baseFunction =  (value)-> 10 + value;
       Integer summation = fc.compute(50, 20, addFunction);
       System.out.println("Summation is " + summation);
       Integer baseResult = fc.baseCompute(20,baseFunction);
       System.out.println("Base computation " + baseResult);
       Integer baseInternal = fc.applyPartialInternalCompute(20, addFunction);
       System.out.println("Base Internal " + baseInternal);
       Integer basepartial= fc.applyPartialExternalCompute(10, addFunction).apply(20);
       System.out.println("Base External " + basepartial);
       Integer baseCurry= fc.applyCurryingCompute(addFunction).apply(10).apply(20);
       System.out.println("Base Curry " + baseCurry); 
     
     
   }  
   

}




Conclusion: Function Currying is a rich technique, In the functional program we use it often to get rid of conceptual duplication. You can use it in Java 8 now so Please use the concept wisely.