PHP generics written in PHP

Table of contents

• How it works
• Installation
• Monomorphization
• Type erasure
• Features
• Tests

How it works

In a nutshell:

• parse generics classes;
• generate concrete classes based on them (you can choose monomorphization or type-erasure);
• autoload concrete classes instead of generics classes.

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.

📘 You can find repository with this example here.

Installation

Require

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

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/"]
        }
    }
}

Monomorphization

A new class is generated for each generic argument combination.

Before monomorphization:

<?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;
    }
}

After monomorphization:

<?php

namespace App;

class BoxForInt {

    private ?int $data = null;
    
    public function set(int $data) : void
    {
        $this->data = $data;
    }
    
    public function get() : ?int
    {
        return $this->data;
    }
}

Command

composer dump-generics

Where in class can generics be used?

<?php

namespace App;

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

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

Where in generic class can parameters be used?

<?php

namespace App;

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

📘 You can read more about monomorphization here.

Type erasure

A new class is generated without generics arguments.

Before type erasure:

<?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;
    }
}

After type erasure:

<?php

namespace App;

class Box {

    private $data = null;
    
    public function set($data) : void
    {
        $this->data = $data;
    }
    
    public function get()
    {
        return $this->data;
    }
}

Command

composer dump-generics --type=type-erasure

Where in class can generics be used?

<?php

namespace App;

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

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

Where in generic class can parameters be used?

<?php

namespace App;

class Test<T,V> extends GenericClass<T> implements GenericInterface<V> { // <-- extends/implements
 
  use GenericTrait<T>; // <-- trait use
 
  private GenericClass<V> $var; // <-- property type
 
  public function test(T|GenericInterface<V> $var): T|GenericClass<V> { // <-- method argument/return type
      
       var_dump($var instanceof GenericInterface<V>); // <-- instanceof          
      
       var_dump(GenericClass<T>::class); // <-- class constants           
       var_dump(GenericClass<T>::CONSTANT); // <-- class constants      
      
       return new GenericClass<V>(); // <-- new           
  }
}

📘 You can read more about type-erasure here.

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'));
   }
}

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?

composer test

How to add test?

• Add directory 00-your-dir-name to ./tests/{monomorphic/type-erased}
• Generate output files and check it

php bin/generate.php monomorphic tests/monomorphic/000-your-dir-name
php bin/generate.php type-erased tests/type-erased/000-your-dir-name