ballerina 学习十五 控制流

ballerina 的控制流没有什么特殊，只是相比一般语言多了一个模式匹配的操作match ，实际上其他语言（erlang elixir rust 中的模式匹配是很强大的）

简单例子

• if/else

import ballerina/io;
function main(string… args) {
    int a = 10;
    int b = 0;
    if (a == 10) {
        io:println("a == 10");
    }

    if (a < b) {
        io:println("a < b");
    } else {
        io:println("a >= b");
    }

    if (b < 0) {
        io:println("b < 0");
    } else if (b > 0) {
        io:println("b > 0");
    } else {
        io:println("b == 0");
    }
}

• while

import ballerina/io;
function main(string… args) {
    int i = 0;
    while (i < 3) {
        io:println(i);
        i = i + 1;
    }

    int j = 0;
    while (j < 5) {
        io:println(j);
        j = j + 1;
        if (j == 3) {
            break;
        }
    }

    int k = 0;
    while (k < 5) {
        if (k < 3) {
            k = k + 1;
            next;
        }

        io:println(k);
        k = k + 1;
    }
}

• match

import ballerina/io;
type KeyNotFoundError {
    string message;
    error? cause;
    string key;
};
map<string?> values = {"key1": "value1", "key2": ()};

function getValue(string key) returns string?|KeyNotFoundError {
    if (!values.hasKey(key)) {
        KeyNotFoundError err = {message: "key '" + key + "' not found", key: key};
        return err;
    } else {
        return values[key];
    }
}

function print(string?|KeyNotFoundError result) {

    match result {
        string value => io:println("value: " + value);
        () => io:println("value is ()");
        KeyNotFoundError e => {
            io:println(e.message);
        }
    }
}

function main(string… args) {
    print(getValue("key1"));
    print(getValue("key2"));
    print(getValue("key3"));
}

• foreach

import ballerina/io;
function main(string… args) {
    io:println("Iterating over a string array:-");
    string[] fruits = ["apple", "banana", "cherry"];
    foreach v in fruits {
        io:println("fruit: " + v);
    }

    io:println("
Iterating over a map:-");
    map<string> words = { a: "apple", b: "banana", c: "cherry" };
    foreach k, v in words {
        io:println("letter: ", k, ", word: ", v);
    }

    io:println("
Iterating over a json object:-");
    json apple = { name: "apple", colors: ["red", "green"], price: 5 };
    foreach j in apple {
        match j {
            string js => {
                io:println("string value: ", js);
            }
            json jx => {
                io:println("non-string value: ", jx);
            }
        }
    }

    io:println("
Iterating over a json array:-");
    json[] colors = check <json[]>apple.colors;
    foreach i, j in colors {
        io:println("color ", i, ": ", j);
    }

    io:println("
Iterating over an xml:-");
    xml book = xml `<book>
                        <name>Sherlock Holmes</name>
                        <author>Sir Arthur Conan Doyle</author>
                    </book>`;
    foreach i, x in book.*.elements(){
        io:println("xml at ", i, ": ", x);
    }

    io:println("
Iterating over an integer range:-");
    int endValue = 10;
    int sum;
    foreach i in [1..endValue] {
        sum = sum + i;
    }
    io:println("summation from 1 to " + endValue + " is " + sum);
}

• match expression ( 类似rust result)

import ballerina/io;
function getAgeCategory(int age) returns string|error {
    if (age < 0) {
        error e = { message: "Invalid" };
        return e;
    } else if (age <= 18) {
        return "Child";
    } else {
        return "Adult";
    }
}

function main(string… args) {

    string ageCategory = getAgeCategory(25) but {
        string s => s,
        error e => e.message
    };
    io:println(ageCategory);

    ageCategory = getAgeCategory(-5) but {
        string s => s,
        error e => e.message
    };
    io:println(ageCategory);
    ageCategory = getAgeCategory(25) but {
        error e => e.message
    };
    io:println(ageCategory);
}

• elvis (

import ballerina/io;
function main(string… args) {
    string|() x = null;
    string matchExprOutput;
    matchExprOutput = x but {
        string s => s,
        () => "value is null"
    };
    io:println("The output from match expression: ", matchExprOutput);

    string elvisOutput = x ?: "value is null";
    io:println("The output from elvis operator: ", elvisOutput);
}

参考资料

https://ballerina.io/learn/by-example/elvis-operator.html
https://ballerina.io/learn/by-example/match-expression.html
https://ballerina.io/learn/by-example/foreach.html
https://ballerina.io/learn/by-example/match.html
https://ballerina.io/learn/by-example/if-else.html
https://ballerina.io/learn/by-example/while.html