Practical use of Constant scalar Expressions in PHP and other languages

Question

In the PHP 5.6 we will have an appeal appointed Constant scalar expressions. The manual provides some examples, but the main focus would be:

• How to use the functionality correctly, without doing so anti-pattern?
• Do other languages implement this type of resource? How it works and is used in them?
• The variables.less Bootstrap would be an example of this concept?

Answer 1

This functionality is not widespread among other languages and there is also no good practice found by common sense, so my opinion is that this new functionality will be very well used by some and by others will be used as a new form of "gambiarra".

Personal opinion:

Good use:

• Calculate default values for method arguments;

In this example, if you want to change the default limit, from 5 to 10, it will change in one place:

<?php
class Foo {
    const DEFAULT_LIMIT = 5;

    public function bar($limit = self::DEFAULT_LIMIT) {
        //code
    }

    public function wonka($limit = self::DEFAULT_LIMIT) {
        //code
    }
}

Misuse:

• Calculation of urls and paths in constants, since this can be used as always, using define() or still creating objects and methods specialized in delivering Paths information for example.

Answer 2

An interesting example are configuration variables.

For example, suppose a scenario where services are consumed, or a basis of approval, or production. These services originate through a URL, which varies depending on the basis in question.

Therefore, listed Urls for these services can be determined by a flag that determines whether the application is in production mode or type approval. See the following example:

// variavel que define ambiente - homogação ou produção
$isHomolog = true;

// constantes para ambiente de homologação
const URL_SERVICO_1_HOMOLOG = "http://homolog1.com";
const URL_SERVICO_2_HOMOLOG = "http://homolog2.com";

// constantes para ambiente de produção
const URL_SERVICO_1_PRODUCTION = "http://production1.com";
const URL_SERVICO_2_PRODUCTION = "http://production2.com";

// constantes utilizadas para os serviços
const URL_SERVICO_1 = $isHomolog ? URL_SERVICO_1_HOMOLOG : URL_SERVICO_1_PRODUCTION;
const URL_SERVICO_2 = $isHomolog ? URL_SERVICO_2_HOMOLOG : URL_SERVICO_2_PRODUCTION;

In this example, constants are defined for the type-approval and production environment. In addition, the variable $isHomolog identifies whether the environment is type-approval (value true) or output (value false). Hence, the service variables used by the system URL_SERVICO_1 and URL_SERVICO_2 has its value conditioned to environment variable $isHomolog.

The advantage of this mechanism is that there is no need to exchange several Urls in the system, depending on its environment: type approval or production.

Note that, the same behavior can-be obtained with variables that are not constant, however the creation of constants ensures that they are not modified by the application. Note that, this possibility of condition in your creation is not allowed after the constants are defined, which ensures their correct use by the system.

Another interesting case is the concatenation of Strings to create a constant. in previous versions of PHP, when trying to create Strings constants based on parts of other Strings, the following message is shown: "PHP Parse error". Now, this is allowed. This is exemplified in link shown in the question itself. See an example below:

// Constant declarations
const PHP = "PHP";
const LOBBY = "Lobby";
const PHPLOBBY = PHP . " " . LOBBY;
echo PHPLOBBY . "\n";
echo "\n=====================\n\n";

Note that the PHPLOBBY constant is defined by a concatenation of Strings. This is useful when constants are used in the system, but so is the concatenation of the system. In order to make the code cleaner, a constant being the result of the concatenation of several times is very useful. This type of assignment for constants is already possible, both in Java and in C# . NET.

Answer 3

Man, I’ll try to explain a good use with an example.

In the PHP code below you can see all variants of scalar expression for PHP encoding, but let’s talk about them. We have the flexibility to use this feature in "Constant declarations", "Class Constant Declarations", "Class Property Declarations", "Argument Declarations", "Static Variable Declarations". As you can see the "PHPLOBBY" in the PHP code below, it is the result of the concatenation of "PHP" and "lobby" with constant empty space between them.

if you try the same approach in previous versions of PHP will give "Parse Exception". The most important thing for me is flexibility in argument statements. We added example of the method in our "Phplobby" class called "is_true". Basically what it does is it does not pass argument that runs to "Constant Scalar Expressions" ( "$ c = self :: NUMBER == 9? "True": "False". )We check whether the constant "Number" is equal to 9 and then assign a value to the variable $ c that we have flexibility to do this only in PHP 5.6 and not in the previous version of PHP.

    <?php
echo "\n===PHPLobby Constant Scalar Expressions ===\n";

// Constant declarations
const PHP = "PHP";
const LOBBY = "Lobby";
const PHPLOBBY = PHP . " " . LOBBY;
echo PHPLOBBY . "\n";
echo "\n=====================\n\n";

// Class Constant Declarations
class PhpLobby {
    const PHP = PHP . " ";
    const LOBBY = self::PHP . LOBBY . "\n";
    const NUMBER = 5 + 4 . "\n";

    // static variable declarations
    static $math = 5 + 4 . "\n";
    static $math2 = 9 - 5 . "\n";
    static $math3 = self::NUMBER - 4 . "\n";

    // class property declarations
    public $php = "PHP " . "Lobby\n";
    public $number = 5 + 4 . "\n";

    // argument declarations
    function is_true($c = self::NUMBER == 9 ? "True" : "Wrong") {
        return $c . "\n";
    }
}
echo PhpLobby::LOBBY;
echo PhpLobby::NUMBER;
echo "\n=====================\n\n";

// Class Property Declarations
$phpLobby = new PhpLobby();
echo $phpLobby->php;
echo $phpLobby->number;
echo "\n=====================\n\n";

// Argument Declarations
echo $phpLobby->is_true(); // weeeeee magic!
echo $phpLobby->is_true(10); // result will be 10 as expected
echo "\n=====================\n\n";

// Static Variable Declarations
echo PhpLobby::$math;
echo PhpLobby::$math2;
echo PhpLobby::$math3;

echo "\n===========================================\n";
?>

Also, here is the list of what can be used with it:

Supported Operation:

1. • - Addition
2. • - Subtraction
3. • - Multiplication
4. / - Division
5. % - Modulus
6. ! - Boolean Negation
7. ~ - Bitwise Negation
8. | - Bitwise OR
9. & - Bitwise AND
10. ^ - Bitwise XOR
11. << - Bitwise Shift Left
12. >> - Bitwise Shift Right
13. . - Concatenation
14. ?: - Ternary Operator
15. <= - Smaller or Equal
16. => - Greater or Equal
17. == - Equal
18. != - Not Equal
19. < - Smaller
20. > - Greater
21. === - Identical
22. !== - Not Identical
23. && / and - Boolean AND
24. || / or - Boolean OR
25. xor - Boolean XOR

Supported Operands

1. 123 - Integers
2. 123.456 - Floats
3. "foo" - Strings
4. LINE - Line Magic Constant
5. FILE - File Magic Constant
6. DIR - Directory Magic Constant
7. TRAIT - Trait Magic Constant
8. METHOD - Method Magic Constant
9. FUNCTION - Function Magic Constant
10. NAMESPACE - Namespace Magic Constant
11. <<
12. <<<'NOWDOC' - NOWDOC string syntax
13. SOME_RANDOM_CONSTANT - Constants
14. class_name::SOME_CONST - Class constants

And if you want to know more, go to their website: https://wiki.php.net/rfc/const_scalar_exprs

I hope I helped! Hugs!