Programmation fonctionnelle pour les développeurs Java, partie 1

Java 8 a introduit les développeurs Java à la programmation fonctionnelle avec des expressions lambda. Cette version Java a effectivement informé les développeurs qu'il ne suffit plus de penser à la programmation Java uniquement du point de vue impératif et orienté objet. Un développeur Java doit également être capable de penser et de coder en utilisant le paradigme fonctionnel déclaratif.

Ce tutoriel présente les bases de la programmation fonctionnelle. Je commencerai par la terminologie, puis nous approfondirons les concepts de programmation fonctionnelle. Je conclurai en vous présentant cinq techniques de programmation fonctionnelle. Les exemples de code dans ces sections vous permettront de démarrer avec des fonctions pures, des fonctions d'ordre supérieur, une évaluation paresseuse, des fermetures et des currying.

La programmation fonctionnelle est à la hausse

L'Institut des ingénieurs électriciens et électroniciens (IEEE) a inclus des langages de programmation fonctionnelle dans ses 25 principaux langages de programmation pour 2018, et Google Trends classe actuellement la programmation fonctionnelle comme plus populaire que la programmation orientée objet.

De toute évidence, la programmation fonctionnelle ne peut être ignorée, mais pourquoi est-elle de plus en plus populaire? Entre autres, la programmation fonctionnelle facilite la vérification de l'exactitude du programme. Il simplifie également la création de programmes concurrents. La concurrence (ou traitement parallèle) est essentielle pour améliorer les performances des applications.

télécharger Obtenir le code Téléchargez le code source des exemples d'applications dans ce didacticiel. Créé par Jeff Friesen pour JavaWorld.

Qu'est-ce que la programmation fonctionnelle?

Les ordinateurs implémentent généralement l'architecture Von Neumann, qui est une architecture informatique largement utilisée basée sur une description de 1945 par le mathématicien et physicien John von Neumann (et d'autres). Cette architecture est biaisée vers la programmation impérative , qui est un paradigme de programmation qui utilise des instructions pour changer l'état d'un programme. C, C ++ et Java sont tous des langages de programmation impératifs.

En 1977, l'éminent informaticien John Backus (remarquable pour son travail sur FORTRAN), a donné une conférence intitulée «La programmation peut-elle être libérée du style von Neumann?». Backus a affirmé que l'architecture de Von Neumann et ses langages impératifs associés sont fondamentalement défectueux, et a présenté un langage de programmation au niveau fonctionnel (FP) comme solution.

Clarifier Backus

Parce que la conférence Backus a été présentée il y a plusieurs décennies, certaines de ses idées pourraient être difficiles à saisir. Le blogueur Tomasz Jaskuła ajoute de la clarté et des notes de bas de page dans son article de blog de janvier 2018.

Concepts et terminologie de la programmation fonctionnelle

La programmation fonctionnelle est un style de programmation dans lequel les calculs sont codifiés en tant que fonctions de programmation fonctionnelle . Ce sont des constructions de type fonction mathématique (par exemple, des fonctions lambda) qui sont évaluées dans des contextes d'expression.

Les langages de programmation fonctionnels sont déclaratifs , ce qui signifie que la logique d'un calcul est exprimée sans décrire son flux de contrôle. Dans la programmation déclarative, il n'y a pas d'instructions. Au lieu de cela, les programmeurs utilisent des expressions pour indiquer à l'ordinateur ce qui doit être fait, mais pas comment accomplir la tâche. Si vous êtes familier avec SQL ou les expressions régulières, alors vous avez une certaine expérience avec le style déclaratif; les deux utilisent des expressions pour décrire ce qui doit être fait, plutôt que des déclarations pour décrire comment le faire.

Un calcul en programmation fonctionnelle est décrit par des fonctions qui sont évaluées dans des contextes d'expression. Ces fonctions ne sont pas les mêmes que les fonctions utilisées dans la programmation impérative, comme une méthode Java qui renvoie une valeur. Au lieu de cela, une fonction de programmation fonctionnelle est comme une fonction mathématique, qui produit une sortie qui ne dépend généralement que de ses arguments. Chaque fois qu'une fonction de programmation fonctionnelle est appelée avec les mêmes arguments, le même résultat est obtenu. On dit que les fonctions de la programmation fonctionnelle présentent une transparence référentielle . Cela signifie que vous pouvez remplacer un appel de fonction par sa valeur résultante sans changer la signification du calcul.

La programmation fonctionnelle favorise l' immuabilité , ce qui signifie que l'état ne peut pas changer. Ce n'est généralement pas le cas dans la programmation impérative, où une fonction impérative peut être associée à state (comme une variable d'instance Java). L'appel de cette fonction à des moments différents avec les mêmes arguments peut entraîner des valeurs de retour différentes car dans ce cas, l'état est mutable , ce qui signifie qu'il change.

Effets secondaires dans la programmation impérative et fonctionnelle

Les changements d'état sont un effet secondaire de la programmation impérative, empêchant la transparence référentielle. Il existe de nombreux autres effets secondaires à connaître, en particulier lorsque vous évaluez s'il faut utiliser le style impératif ou fonctionnel dans vos programmes.

Un effet secondaire courant dans la programmation impérative est lorsqu'une instruction d'affectation mute une variable en modifiant sa valeur stockée. Les fonctions de la programmation fonctionnelle ne prennent pas en charge les affectations de variables. Étant donné que la valeur initiale d'une variable ne change jamais, la programmation fonctionnelle élimine cet effet secondaire.

Un autre effet secondaire courant se produit lors de la modification du comportement d'une fonction impérative basée sur une exception levée, qui est une interaction observable avec l'appelant. Pour plus d'informations, consultez la discussion Stack Overflow, "Pourquoi la levée d'une exception est-elle un effet secondaire?"

Un troisième effet secondaire courant se produit lorsqu'une opération d'E / S entre du texte qui ne peut pas être non lu ou produit du texte qui ne peut pas être non écrit. Consultez la discussion Stack Exchange "Comment IO peut-il provoquer des effets secondaires dans la programmation fonctionnelle?" pour en savoir plus sur cet effet secondaire.

L'élimination des effets secondaires facilite la compréhension et la prédiction du comportement informatique. Cela permet également de rendre le code plus adapté au traitement parallèle, ce qui améliore souvent les performances des applications. Bien qu'il y ait des effets secondaires dans la programmation fonctionnelle, ils sont généralement moins nombreux que dans la programmation impérative. L'utilisation de la programmation fonctionnelle peut vous aider à écrire du code plus facile à comprendre, à maintenir et à tester, et qui est également plus réutilisable.

Origines (et initiateurs) de la programmation fonctionnelle

La programmation fonctionnelle est issue du calcul lambda, qui a été introduit par Alonzo Church. Une autre origine est la logique combinatoire, qui a été introduite par Moses Schönfinkel puis développée par Haskell Curry.

Programmation orientée objet versus programmation fonctionnelle

J'ai créé une application Java qui contraste les approches de programmation fonctionnelle impérative, orientée objet et déclarative de l'écriture de code. Étudiez le code ci-dessous, puis je soulignerai les différences entre les deux exemples.

Listing 1. Employees.java

import java.util.ArrayList; import java.util.List; public class Employees { static class Employee { private String name; private int age; Employee(String name, int age) { this.name = name; this.age = age; } int getAge() { return age; } @Override public String toString() { return name + ": " + age; } } public static void main(String[] args) { List employees = new ArrayList(); employees.add(new Employee("John Doe", 63)); employees.add(new Employee("Sally Smith", 29)); employees.add(new Employee("Bob Jone", 36)); employees.add(new Employee("Margaret Foster", 53)); printEmployee1(employees, 50); System.out.println(); printEmployee2(employees, 50); } public static void printEmployee1(List employees, int age) { for (Employee emp: employees) if (emp.getAge() < age) System.out.println(emp); } public static void printEmployee2(List employees, int age) { employees.stream() .filter(emp -> emp.age  System.out.println(emp)); } }

Le listing 1 révèle une Employeesapplication qui crée quelques Employeeobjets, puis imprime une liste de tous les employés de moins de 50 ans. Ce code illustre à la fois les styles de programmation orientés objet et fonctionnels.

The printEmployee1() method reveals the imperative, statement-oriented approach. As specified, this method iterates over a list of employees, compares each employee's age against an argument value, and (if the age is less than the argument), prints the employee's details.

The printEmployee2() method reveals the declarative, expression-oriented approach, in this case implemented with the Streams API. Instead of imperatively specifying how to print the employees (step-by-step), the expression specifies the desired outcome and leaves the details of how to do it to Java. Think of filter() as the functional equivalent of an if statement, and forEach() as functionally equivalent to the for statement.

You can compile Listing 1 as follows:

javac Employees.java

Use the following command to run the resulting application:

java Employees

The output should look something like this:

Sally Smith: 29 Bob Jone: 36 Sally Smith: 29 Bob Jone: 36

Functional programming examples

In the next sections, we'll explore five core techniques used in functional programming: pure functions, higher-order functions, lazy evaluation, closures, and currying. Examples in this section are coded in JavaScript because its simplicity, relative to Java, will allow us to focus on the techniques. In Part 2 we'll revisit these same techniques using Java code.

Listing 2 presents the source code to RunScript, a Java application that uses Java's Scripting API to facilitate running JavaScript code. RunScript will be the base program for all of the forthcoming examples.

Listing 2. RunScript.java

import java.io.FileReader; import java.io.IOException; import javax.script.ScriptEngine; import javax.script.ScriptEngineManager; import javax.script.ScriptException; import static java.lang.System.*; public class RunScript { public static void main(String[] args) { if (args.length != 1) { err.println("usage: java RunScript script"); return; } ScriptEngineManager manager = new ScriptEngineManager(); ScriptEngine engine = manager.getEngineByName("nashorn"); try { engine.eval(new FileReader(args[0])); } catch (ScriptException se) { err.println(se.getMessage()); } catch (IOException ioe) { err.println(ioe.getMessage()); } } }

The main() method in this example first verifies that a single command-line argument (the name of a script file) has been specified. Otherwise, it displays usage information and terminates the application.

Assuming the presence of this argument, main() instantiates the javax.script.ScriptEngineManager class. ScriptEngineManager is the entry-point into Java's Scripting API.

Next, the ScriptEngineManager object's ScriptEngine getEngineByName(String shortName) method is called to obtain a script engine corresponding to the desired shortName value. Java 10 supports the Nashorn script engine, which is obtained by passing "nashorn" to getEngineByName(). The returned object's class implements the javax.script.ScriptEngine interface.

ScriptEngine declares several eval() methods for evaluating a script. main() invokes the Object eval(Reader reader) method to read the script from its java.io.FileReader object argument and (assuming that java.io.IOException isn't thrown) then evaluate the script. This method returns any script return value, which I ignore. Also, this method throws javax.script.ScriptException when an error occurs in the script.

Compile Listing 2 as follows:

javac RunScript.java

I'll show you how to run this application after I have presented the first script.

Functional programming with pure functions

A pure function is a functional programming function that depends only on its input arguments and no external state. An impure function is a functional programming function that violates either of these requirements. Because pure functions have no interaction with the outside world (apart from calling other pure functions), a pure function always returns the same result for the same arguments. Pure functions also have no observable side effects.

Can a pure function perform I/O?

If I/O is a side effect, can a pure function perform I/O? The answer is yes. Haskell uses monads to address this problem. See "Pure Functions and I/O" for more about pure functions and I/O.

Pure functions versus impure functions

The JavaScript in Listing 3 contrasts an impure calculatebonus() function with a pure calculatebonus2() function.

Listing 3. Comparing pure vs impure functions (script1.js)

// impure bonus calculation var limit = 100; function calculatebonus(numSales) { return(numSales > limit) ? 0.10 * numSales : 0 } print(calculatebonus(174)) // pure bonus calculation function calculatebonus2(numSales) { return (numSales > 100) ? 0.10 * numSales : 0 } print(calculatebonus2(174))

calculatebonus() is impure because it accesses the external limit variable. In contrast, calculatebonus2() is pure because it obeys both requirements for purity. Run script1.js as follows:

java RunScript script1.js

Here's the output you should observe:

17.400000000000002 17.400000000000002

Suppose calculatebonus2() was refactored to return calculatebonus(numSales). Would calculatebonus2() still be pure? The answer is no: when a pure function invokes an impure function, the "pure function" becomes impure.

When no data dependency exists between pure functions, they can be evaluated in any order without affecting the outcome, making them suitable for parallel execution. This is one of functional programming's benefits.

More about impure functions

Not all functional programming functions need to be pure. As Functional Programming: Pure Functions explains, it is possible (and sometimes desirable) to "separate the pure, functional, value based core of your application from an outer, imperative shell."

Functional programming with higher-order functions

A higher-order function is a mathematical function that receives functions as arguments, returns a function to its caller, or both. One example is calculus's differential operator, d/dx, which returns the derivative of function f.

First-class functions are first-class citizens

Closely related to the mathematical higher-order function concept is the first-class function, which is a functional programming function that takes other functional programming functions as arguments and/or returns a functional programming function. First-class functions are first-class citizens because they can appear wherever other first-class program entities (e.g., numbers) can, including being assigned to a variable or being passed as an argument to or returned from a function.

The JavaScript in Listing 4 demonstrates passing anonymous comparison functions to a first-class sorting function.

Listing 4. Passing anonymous comparison functions (script2.js)

function sort(a, cmp) { for (var pass = 0; pass 
    
      pass; i--) if (cmp(a[i], a[pass]) < 0) { var temp = a[i] a[i] = a[pass] a[pass] = temp } } var a = [22, 91, 3, 45, 64, 67, -1] sort(a, function(i, j) { return i - j; }) a.forEach(function(entry) { print(entry) }) print('\n') sort(a, function(i, j) { return j - i; }) a.forEach(function(entry) { print(entry) }) print('\n') a = ["X", "E", "Q", "A", "P"] sort(a, function(i, j) { return i 
     
       j; }) a.forEach(function(entry) { print(entry) }) print('\n') sort(a, function(i, j) { return i > j ? -1 : i < j; }) a.forEach(function(entry) { print(entry) })
     
    

In this example, the initial sort() call receives an array as its first argument, followed by an anonymous comparison function. When called, the anonymous comparison function executes return i - j; to achieve an ascending sort. By reversing i and j, the second comparison function achieves a descending sort. The third and fourth sort() calls receive anonymous comparison functions that are slightly different in order to properly compare string values.

Run the script2.js example as follows:

java RunScript script2.js

Here's the expected output:

-1 3 22 45 64 67 91 91 67 64 45 22 3 -1 A E P Q X X Q P E A

Filter and map

Functional programming languages typically provide several useful higher-order functions. Two common examples are filter and map.

  • A filter processes a list in some order to produce a new list containing exactly those elements of the original list for which a given predicate (think Boolean expression) returns true.
  • A map applies a given function to each element of a list, returning a list of results in the same order.

JavaScript supports filtering and mapping functionality via the filter() and map() higher-order functions. Listing 5 demonstrates these functions for filtering out odd numbers and mapping numbers to their cubes.

Listing 5. Filtering and mapping (script3.js)

print([1, 2, 3, 4, 5, 6].filter(function(num) { return num % 2 == 0 })) print('\n') print([3, 13, 22].map(function(num) { return num * 3 }))

Run the script3.js example as follows:

java RunScript script3.js

You should observe the following output:

2,4,6 9,39,66

Reduce

Another common higher-order function is reduce, which is more commonly known as a fold. This function reduces a list to a single value.

Listing 6 uses JavaScript's reduce() higher-order function to reduce an array of numbers to a single number, which is then divided by the array's length to obtain an average.

Listing 6. Reducing an array of numbers to a single number (script4.js)

var numbers = [22, 30, 43] print(numbers.reduce(function(acc, curval) { return acc + curval }) / numbers.length)

Run Listing 6's script (in script4.js) as follows:

java RunScript script4.js

You should observe the following output:

31.666666666666668

You might think that the filter, map, and reduce higher-order functions obviate the need for if-else and various looping statements, and you would be right. Their internal implementations take care of decisions and iteration.

A higher-order function uses recursion to achieve iteration. A recursive function invokes itself, allowing an operation to repeat until it reaches a base case. You can also leverage recursion to achieve iteration in your functional code.

Functional programming with lazy evaluation

Another important functional programming feature is lazy evaluation (also known as nonstrict evaluation), which is the deferral of expression evaluation for as long as possible. Lazy evaluation offers several benefits, including these two:

  • Expensive (timewise) calculations can be deferred until they're absolutely necessary.
  • Unbounded collections are possible. They'll keep delivering elements for as long as they're requested to do so.

Lazy evaluation is integral to Haskell. It won't calculate anything (including a function's arguments before the function is called) unless it's strictly necessary to do so.

Java's Streams API capitalizes on lazy evaluation. A stream's intermediate operations (e.g., filter()) are always lazy; they don't do anything until a terminal operation (e.g., forEach()) is executed.

Although lazy evaluation is an important part of functional languages, even many imperative languages provide builtin support for some forms of laziness. For example, most programming languages support short-circuit evaluation in the context of the Boolean AND and OR operators. These operators are lazy, refusing to evaluate their right-hand operands when the left-hand operand is false (AND) or true (OR).

Listing 7 is an example of lazy evaluation in a JavaScript script.

Listing 7. Lazy evaluation in JavaScript (script5.js)

var a = false && expensiveFunction("1") var b = true && expensiveFunction("2") var c = false || expensiveFunction("3") var d = true || expensiveFunction("4") function expensiveFunction(id) { print("expensiveFunction() called with " + id) }

Run the code in script5.js as follows:

java RunScript script5.js

You should observe the following output:

expensiveFunction() called with 2 expensiveFunction() called with 3

Lazy evaluation is often combined with memoization, an optimization technique used primarily to speed up computer programs by storing the results of expensive function calls and returning the cached result when the same inputs reoccur.

Because lazy evaluation doesn't work with side effects (such as code that produces exceptions and I/O), imperative languages mainly use eager evaluation (also known as strict evaluation), where an expression is evaluated as soon as it's bound to a variable.

More about lazy evaluation and memoization

A Google search will reveal many useful discussions of lazy evaluation with or without memoization. One example is "Optimizing your JavaScript with functional programming."

Functional programming with closures

First-class functions are associated with the concept of a closure, which is a persistent scope that holds onto local variables even after the code execution has left the block in which the local variables were defined.

Crafting closures

Operationally, a closure is a record that stores a function and its environment. The environment maps each of the function's free variables (variables used locally, but defined in an enclosing scope) with the value or reference to which the variable's name was bound when the closure was created. It lets the function access those captured variables through the closure's copies of their values or references, even when the function is invoked outside their scope.

To help clarify this concept, Listing 8 presents a JavaScript script that introduces a simple closure. The script is based on the example presented here.

Listing 8. A simple closure (script6.js)

function add(x) { function partialAdd(y) { return y + x } return partialAdd } var add10 = add(10) var add20 = add(20) print(add10(5)) print(add20(5))

Listing 8 defines a first-class function named add() with a parameter x and a nested function partialAdd(). The nested function partialAdd() has access to x because x is in add()'s lexical scope. Function add() returns a closure that contains a reference to partialAdd() and a copy of the environment around add(), in which x has the value assigned to it in a specific invocation of add().

Because add() returns a value of function type, variables add10 and add20 also have function type. The add10(5) invocation returns 15 because the invocation assigns 5 to parameter y in the call to partialAdd(), using the saved environment for partialAdd() where x is 10. The add20(5) invocation returns 25 because, although it also assigns 5 to y in the call to partialAdd(), it's now using another saved environment for partialAdd() where x is 20. Thus, while add10() and add20() use the same function partialAdd(), the associated environments differ and invoking the closures will bind x to two different values in the two invocations, evaluating the function to two different results.

Run Listing 8's script (in script6.js) as follows:

java RunScript script6.js

You should observe the following output:

15 25

Functional programming with currying

Currying is a way to translate the evaluation of a multi-argument function into the evaluation of an equivalent sequence of single-argument functions. For example, a function takes two arguments: x and y. Currying transforms the function into taking only x and returning a function that takes only y. Currying is related to but is not the same as partial application, which is the process of fixing a number of arguments to a function, producing another function of smaller arity.

Listing 9 presents a JavaScript script that demonstrates currying.

Listing 9. Currying in JavaScript (script7.js)

function multiply(x, y) { return x * y } function curried_multiply(x) { return function(y) { return x * y } } print(multiply(6, 7)) print(curried_multiply(6)(7)) var mul_by_4 = curried_multiply(4) print(mul_by_4(2))

The script presents a noncurried two-argument multiply() function, followed by a first-class curried_multiply() function that receives multiplicand argument x and returns a closure containing a reference to an anonymous function (that receives multiplier argument y) and a copy of the environment around curried_multiply(), in which x has the value assigned to it in an invocation of curried_multiply().

The rest of the script first invokes multiply() with two arguments and prints the result. It then invokes curried_multiply() in two ways:

  • curried_multiply(6)(7) results in curried_multiply(6) executing first. The returned closure executes the anonymous function with the closure's saved x value 6 being multiplied by 7.
  • var mul_by_4 = curried_multiply(4) executes curried_multiply(4) and assigns the closure to mul_by_4. mul_by_4(2) executes the anonymous function with the closure's 4 value and the passed argument 2.

Run Listing 9's script (in script7.js) as follows:

java RunScript script7.js

You should observe the following output:

42 42 8

Why use currying?

In his blog post "Why curry helps," Hugh Jackson observes that "little pieces can be configured and reused with ease, without clutter." Quora's "What are the advantages of currying in functional programming?" describes currying as "a cheap form of dependency injection," that eases the process of mapping/filtering/folding (and higher order functions generally). This Q&A also notes that currying "helps us create abstract functions."

In conclusion

In this tutorial you've learned some basics of functional programming. We've used examples in JavaScript to study five core functional programming techniques, which we'll further explore using Java code in Part 2. In addition to touring Java 8's functional programming capabilities, the second half of this tutorial will help you begin to think functionally, by converting an example of object-oriented Java code to its functional equivalent.

Learn more about functional programming

I found the book Introduction to Functional Programming (Richard Bird and Philip Wadler, Prentice Hall International Series in Computing Science, 1992) helpful in learning the basics of functional programming.

This story, "Functional programming for Java developers, Part 1" was originally published by JavaWorld .