PHP generics written in PHP

Require

• PHP >= 7.4
• Composer (PSR-4 Autoload)

Table of contents

• How it works
• Quick start
• Example
• Features
• Tests

How it works

• parse generics classes;
• generate concrete classes based on them;
• say to "composer autoload" to load files from directory with generated classes first and then load classes from main directory.

Quick start

Install library

composer require mrsuh/php-generics

Add directory("cache/") to composer autoload PSR-4 for generated classes. It should be placed before the main directory.

composer.json

{
    "autoload": { 
        "psr-4": {
            "App\\": ["cache/","src/"]
        }
    }
}

Generate concrete classes from generic classes with composer dump-generics command

composer dump-generics -vv

Generate vendor/autoload.php with composer dump-autoload command

composer dump-autoload

Example

!! You can find repository with this example here.

For example, you need to add several PHP files:

• generic class Box
• class Usage for use generic class
• script with composer autoload and Usage class

src/Box.php

<?php

namespace App;

class Box<T> {

    private ?T $data = null;

    public function set(T $data): void {
        $this->data = $data;
    }

    public function get(): ?T {
        return $this->data;
    }
}

src/Usage.php

<?php

namespace App;

class Usage {

    public function run(): void
    {
        $stringBox = new Box<string>();
        $stringBox->set('cat');
        var_dump($stringBox->get()); // string "cat"

        $intBox = new Box<int>();
        $intBox->set(1);
        var_dump($intBox->get()); // integer 1
    }
}

bin/test.php

<?php

require_once __DIR__ . '/../vendor/autoload.php';

use App\Usage;

$usage = new Usage();
$usage->run();

Generate concrete classes from generic classes with composer dump-generics command

composer dump-generics -vv

What the composer dump-generics command does?

• finds all generic uses in classes (src/Usage.php for example).
• generates concrete classes from generic classes with unique names based on name and arguments of generic class.
• replaces generic class names to concrete class names in places of use.

In this case should be generated:

• 2 concrete classes of generics BoxForInt and BoxForString;
• 1 concrete class Usage with replaced generics class names to concrete class names.

Generate vendor/autoload.php with composer dump-autoload command

composer dump-autoload

Run bin/test.php script

php bin/test.php

Composer autoload first checks the "cache" directory and then the "src" directory to load the classes.

Features

What syntax is used?

The RFC does not define a specific syntax so i took this one implemented by Nikita Popov

Syntax example:

<?php

namespace App;

class Generic<in T: Iface = int, out V: Iface = string> {
  
   public function test(T $var): V {
  
   }
}

Syntax problems

I had to upgrade nikic/php-parser for parse code with new syntax.
You can see here the grammar changes that had to be made for support generics.

Parser use PHP implementation of YACC.
The YACC(LALR) algorithm and current PHP syntax make it impossible to describe the full syntax of generics due to collisions.

Collision example:

<?php

const FOO = 'FOO';
const BAR = 'BAR';

var_dump(new \DateTime<FOO,BAR>('now')); // is it generic?
var_dump( (new \DateTime < FOO) , ( BAR > 'now') ); // no, it is not

Solution options

Therefore, nested generics are not currently supported.

<?php

namespace App;

class Usage {
   public function run() {
       $map = new Map<Key<int>, Value<string>>();//not supported
   }
}

Parameter names have not special restrictions

<?php

namespace App;

class GenericClass<T, varType, myCoolLongParaterName> {
   private T $var1;
   private varType $var2;
   private myCoolLongParaterName $var3;   
}

Several generic parameters support

<?php

namespace App;

class Map<keyType, valueType> {
  
   private array $map;
  
   public function set(keyType $key, valueType $value): void {
       $this->map[$key] = $value;
   }
  
   public function get(keyType $key): ?valueType {
       return $this->map[$key] ?? null;
   }
}

Default generic parameter support

<?php

namespace App;

class Map<keyType = string, valueType = int> {
  
   private array $map = [];
  
   public function set(keyType $key, valueType $value): void {
       $this->map[$key] = $value;
   }
  
   public function get(keyType $key): ?valueType {
       return $this->map[$key] ?? null;
   }
}

<?php

namespace App;

class Usage {
   public function run() {
       $map = new Map<>();//be sure to add "<>"
       $map->set('key', 1);
       var_dump($map->get('key'));
   }
}

Where in class can generics be used?

• extends
• implements
• trait use
• property type
• method argument type
• method return type
• instanceof
• new
• class constants

An example of class that uses generics:

<?php

namespace App;

use App\Entity\Cat;
use App\Entity\Bird;
use App\Entity\Dog;

class Test extends GenericClass<Cat> implements GenericInterface<Bird> {
 
  use GenericTrait<Dog>;
 
  private GenericClass<int>|GenericClass<Dog> $var;
 
  public function test(GenericInterface<int>|GenericInterface<Dog> $var): GenericClass<string>|GenericClass<Bird> {
      
       var_dump($var instanceof GenericInterface<int>);
      
       var_dump(new GenericClass<int>::class);
      
       var_dump(new GenericClass<array>::CONSTANT);
      
       return new GenericClass<float>();
  }
}

Where in generic class can parameters be used?

• extends
• implements
• trait use
• property type
• method argument type
• method return type
• instanceof
• new
• class constants

And example of generic class:

<?php

namespace App;

class Test<T,V> extends GenericClass<T> implements GenericInterface<V> {
 
  use GenericTrait<T>;
  use T;
 
  private T|GenericClass<V> $var;
 
  public function test(T|GenericInterface<V> $var): T|GenericClass<V> {
      
       var_dump($var instanceof GenericInterface<V>);
      
       var_dump($var instanceof T);
      
       var_dump(new GenericClass<T>::class);
      
       var_dump(T::class);
      
       var_dump(new GenericClass<T>::CONSTANT);
      
       var_dump(T::CONSTANT);
      
       $obj1 = new T();
       $obj2 = new GenericClass<V>();
      
       return $obj2;
  }
}

How fast is it?

All concrete classes are pre-generated and can be cached(should not affect performance).

Generating many concrete classes should negatively impact performance when:

• resolves concrete classes;
• storing concrete classes in memory;
• type checking for each concrete class.

I think it's all individual for a specific case.

Doesn't work without composer autoload

Autoload magic of concrete classes works with composer autoload only.
Nothing will work because of syntax error if you include file by "require"

Reflection

PHP does type checks in runtime.
Therefore, all generics arguments must me available through reflection in runtime.
It can't be, because information about generics arguments is erased after concrete classes are generated.

Tests

How to run tests?

php bin/test.php

How to add test?

• Add directory 00-your-dir-name to ./tests
• Generate output files and check it

php bin/generate.php tests/000-your-dir-name/input tests/000-your-dir-name/output 'Test\'