函数式编程简介

过去

Functional Programming鼻祖 => Lisp

关于编程语言历史可以参考ComputerLanguagesChart

Lisp = 第二悠久的仍被广泛使用的高级编程语言

关于Lisp语言更多介绍可以参考LISP - 维基百科

Lisp现代方言主要有Clojure、Common Lisp、Scheme

brew install clojure

clj

(println "Hello, World!")
;; Hello, World!
;; nil

(def a 42)
(+ a 8)
;; 50

(->> (range 10)
     (map inc)
     (filter even?))
;; (2 4 6 8 10)

关于上述Clojure代码更多介绍可以参考What does ->> mean in Clojure?

特征

First-class Functions

This means functions can be treated as values. They can be assigned as values, passed into functions, and returned from functions

;; clojure
;;    name   params         body
;;    -----  ------  -------------------
(defn greet  [name]  (str "Hello, " name) )
(greet "students")
;; "Hello, students"

(defn adder [x]
  (fn [a] (+ x a)))
(def add-five (adder 5))
(add-five 100)
;; 105

Immutable State

;; clojure
(let [my-vector [1 2 3 4]
      my-map {:fred "ethel"}
      my-list (list 4 3 2 1)]
  (list
    (conj my-vector 5)
    (assoc my-map :ricky "lucy")
    (conj my-list 5)
    my-vector
    my-map
    my-list))
;; ([1 2 3 4 5] {:fred "ethel", :ricky "lucy"} (5 4 3 2 1) [1 2 3 4] {:fred "ethel"} (4 3 2 1))

关于Immutable的更多介绍可以参考Why Immutable?

要求

Pure Function, no Side Effects

Pure Function: 相同输入的情况下输出总是相同
No Side Effects: 不会修改函数外部的任何变量

Expression, no Statement

Expression: 单纯的运算过程, 总是有返回值
Statement: 执行某种操作, 没有返回值

发展

Python

# python
a = [(1, 2), (4, 1), (9, 10), (13, -3)]
a.sort(key=lambda x: x[0])
print(a)
# [(1, 2), (4, 1), (9, 10), (13, -3)]

a = [(1, 2), (4, 1), (9, 10), (13, -3)]
b = sorted(a)
print(b)
# [(1, 2), (4, 1), (9, 10), (13, -3)]

JavaScript

cnpm i --save lodash

var _ = require('lodash');

var r = _.map([1, 2, 3], function (el) {
    return el * 2
});
console.log(r); // return [ 2, 4, 6 ]

关于Lodash的更多介绍可以参考Lodash - Wikipedia / Lodash Documentation

PHP

composer create-project laravel/laravel php-fp --prefer-dist "5.5.*"

cd php-fp

php artisan tinker

$array = [
    ['name' => 'Desk'],
    ['name' => 'Table'],
    ['name' => 'Chair'],
];

$sorted = array_values(array_sort($array, function ($value) {
    return $value['name'];
}));
/*
    [
        ['name' => 'Chair'],
        ['name' => 'Desk'],
        ['name' => 'Table'],
    ]
*/

关于Laravel辅助函数的更多介绍可以参考辅助函数

Java

mkdir java-fp && cd java-fp

gradle init --type java-application

vim ./src/main/java/Dish.java

import java.util.Arrays;
import java.util.List;

public class Dish {
    private final String name;
    private final boolean vegetarian;
    private final int calories;
    private final Type type;

    public Dish(String name, boolean vegetarian, int calories, Type type) {
        this.name = name;
        this.vegetarian = vegetarian;
        this.calories = calories;
        this.type = type;
    }

    public String getName() {
        return name;
    }

    public boolean isVegetarian() {
        return vegetarian;
    }

    public int getCalories() {
        return calories;
    }

    public Type getType() {
        return type;
    }

    public enum Type {MEAT, FISH, OTHER}

    @Override
    public String toString() {
        return name;
    }

    public static final List<Dish> menu =
            Arrays.asList(new Dish("pork", false, 800, Dish.Type.MEAT),
                    new Dish("beef", false, 700, Dish.Type.MEAT),
                    new Dish("chicken", false, 400, Dish.Type.MEAT),
                    new Dish("french fries", true, 530, Dish.Type.OTHER),
                    new Dish("rice", true, 350, Dish.Type.OTHER),
                    new Dish("season fruit", true, 120, Dish.Type.OTHER),
                    new Dish("pizza", true, 550, Dish.Type.OTHER),
                    new Dish("prawns", false, 400, Dish.Type.FISH),
                    new Dish("salmon", false, 450, Dish.Type.FISH));
}

./src/main/java/App.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

import java.util.stream.Collectors;

public class App {
    public static void main(String... args) {
        // Java 7
        getLowCaloricDishesNamesInJava7(Dish.menu).forEach(System.out::println);

        System.out.println("---");

        // Java 8
        getLowCaloricDishesNamesInJava8(Dish.menu).forEach(System.out::println);

    }

    public static List<String> getLowCaloricDishesNamesInJava7(List<Dish> dishes) {
        List<Dish> lowCaloricDishes = new ArrayList<>();
        for (Dish d : dishes) {
            if (d.getCalories() < 400) {
                lowCaloricDishes.add(d);
            }
        }
        List<String> lowCaloricDishesName = new ArrayList<>();
        Collections.sort(lowCaloricDishes, new Comparator<Dish>() {
            @Override
            public int compare(Dish d1, Dish d2) {
                return Integer.compare(d1.getCalories(), d2.getCalories());
            }
        });
        for (Dish d : lowCaloricDishes) {
            lowCaloricDishesName.add(d.getName());
        }
        return lowCaloricDishesName;
    }

    public static List<String> getLowCaloricDishesNamesInJava8(List<Dish> dishes) {
        return dishes.stream()
                .filter(d -> d.getCalories() < 400)
                .sorted(Comparator.comparing(Dish::getCalories))
                .map(Dish::getName)
                .collect(Collectors.toList());
    }
}

gradle run

更多示例代码可以参考Java8InAction

价值

Method Chaining

Pure functions are easier to reason about

function-programming-introduction-01.png

Strategy Pattern: Define a family of algorithms, encapsulate each one, and make them interchangeable. [The] Strategy [pattern] lets the algorithm vary independently from clients that use it.

public class App {
    public static void main(String[] args) {
        // old school
        Validator v1 = new Validator(new IsNumeric());
        System.out.println(v1.validate("aaaa"));
        Validator v2 = new Validator(new IsAllLowerCase());
        System.out.println(v2.validate("bbbb"));

        // with lambdas
        Validator v3 = new Validator((String s) -> s.matches("\\d+"));
        System.out.println(v3.validate("aaaa"));
        Validator v4 = new Validator((String s) -> s.matches("[a-z]+"));
        System.out.println(v4.validate("bbbb"));
    }

    interface ValidationStrategy {
        public boolean execute(String s);
    }

    static private class IsAllLowerCase implements ValidationStrategy {
        @Override
        public boolean execute(String s) {
            return s.matches("[a-z]+");
        }
    }

    static private class IsNumeric implements ValidationStrategy {
        @Override
        public boolean execute(String s) {
            return s.matches("\\d+");
        }
    }

    static private class Validator {
        private final ValidationStrategy strategy;

        public Validator(ValidationStrategy v) {
            this.strategy = v;
        }

        public boolean validate(String s) {
            return strategy.execute(s);
        }
    }
}

关于策略模式更多介绍可以参考策略模式 - Wikipedia

Chain of Responsibility: Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

import java.util.function.Function;
import java.util.function.UnaryOperator;

public class App {
    public static void main(String[] args) {
        ProcessingObject<String> p1 = new HeaderTextProcessing();
        ProcessingObject<String> p2 = new SpellCheckerProcessing();
        p1.setSuccessor(p2);
        String result1 = p1.handle("Aren't labdas really sexy?!!");
        System.out.println(result1);

        UnaryOperator<String> headerProcessing =
                (String text) -> "From Raoul, Mario and Alan: " + text;
        UnaryOperator<String> spellCheckerProcessing =
                (String text) -> text.replaceAll("labda", "lambda");
        Function<String, String> pipeline = headerProcessing.andThen(spellCheckerProcessing);
        String result2 = pipeline.apply("Aren't labdas really sexy?!!");
        System.out.println(result2);
    }

    static private abstract class ProcessingObject<T> {
        protected ProcessingObject<T> successor;

        public void setSuccessor(ProcessingObject<T> successor) {
            this.successor = successor;
        }

        public T handle(T input) {
            T r = handleWork(input);
            if (successor != null) {
                return successor.handle(r);
            }
            return r;
        }

        abstract protected T handleWork(T input);
    }

    static private class HeaderTextProcessing
            extends ProcessingObject<String> {
        @Override
        public String handleWork(String text) {
            return "From Raoul, Mario and Alan: " + text;
        }
    }

    static private class SpellCheckerProcessing
            extends ProcessingObject<String> {
        @Override
        public String handleWork(String text) {
            return text.replaceAll("labda", "lambda");
        }
    }
}

关于责任链模式更多介绍可以参考责任链模式 - Wikipedia

Parallel Programming

关于Parallel和Concurrent的区别可以参考并发与并行的区别？

import java.util.stream.Stream;

public class App {
    public static void main(String[] args) {
        System.out.println(iterativeSum(100));

        System.out.println(sequentialSum(100));

        System.out.println(parallelSum(100));
    }

    public static long iterativeSum(long n) {
        long result = 0;
        for (long i = 0; i <= n; i++) {
            result += i;
        }
        return result;
    }

    public static long sequentialSum(long n) {
        return Stream.iterate(1L, i -> i + 1).limit(n).reduce(Long::sum).get();
    }

    public static long parallelSum(long n) {
        return Stream.iterate(1L, i -> i + 1).limit(n).parallel().reduce(Long::sum).get();
    }
}

function-programming-introduction-02.png

关于并行数据处理的更多介绍可以参考《Java8实战》-第七章笔记（并行数据处理与性能）

练习

给定一个列表List<value> 比如{1, 4, 9} 构造一个List<List<Integer>> 它的成员都是类表{1, 4, 9}的子集我们暂时不考虑元素的顺序

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class App {
    public static void main(String[] args) {
        List<List<Integer>> subs = subsets(Arrays.asList(1, 4, 9));
        subs.forEach(System.out::println);
    }

    public static List<List<Integer>> subsets(List<Integer> l) {
        if (l.isEmpty()) {
            List<List<Integer>> ans = new ArrayList<>();
            ans.add(Collections.emptyList());
            return ans;
        }
        Integer first = l.get(0);
        List<Integer> rest = l.subList(1, l.size());
        List<List<Integer>> subans = subsets(rest);
        List<List<Integer>> subans2 = insertAll(first, subans);
        return concat(subans, subans2);
    }

    public static List<List<Integer>> insertAll(Integer first, List<List<Integer>> lists) {
        List<List<Integer>> result = new ArrayList<>();
        for (List<Integer> l : lists) {
            List<Integer> copyList = new ArrayList<>();
            copyList.add(first);
            copyList.addAll(l);
            result.add(copyList);
        }
        return result;
    }

    static List<List<Integer>> concat(List<List<Integer>> a, List<List<Integer>> b) {
        List<List<Integer>> r = new ArrayList<>(a);
        r.addAll(b);
        return r;
    }
}

给定一个列表List<value> 比如{1, 4, 5, 9, 8, 1, 3, 6, 7} 构造一个List<List<Integer>> 形如{{1, 4, 5}, {9, 8}, {1, 3}, {6, 7}} 即以0~5, 6~7, 8~9划分成连续的分段

函数式编程简介

函数式编程简介

目录

过去

特征

First-class Functions

Immutable State

要求

Pure Function, no Side Effects

Expression, no Statement

发展

Python

JavaScript

PHP

Java

价值

Method Chaining

Parallel Programming

练习

参考

友情链接更多精彩内容