phpgenericsfactory-patternphpstanpsalm-php

How to write generics for factories using psalm and phpstan


I'm trying out phpstan and psalm for php and I would like to write a class that can take different type of objects and return the right one based on the factory to call.

What I'm trying to achieve is that if I pass an object of type A to the Transformer, the compiler knows that a SuperA will be returned.

While I can go without errors in psalm (though I still get SuperA|SuperB instead of the right object), I got an error on what I'm passing in phpstan.

https://phpstan.org/r/4fce6f46-7aea-4f73-8259-df895910f064

https://psalm.dev/r/352e64ea95

Is there a way to do it?


Solution

  • So you want to get SuperA based on A and SuperB based on B.

    I'd connect A+SuperA and B+SuperB together like this: https://phpstan.org/r/28e4e6ec-887b-4735-9b34-c034b4fa04ec

    /**
     * @template TSuper of Super
     */
    interface Common
    {
    }
    
    /**
     * @implements Common<SuperA>
     */ 
    class A implements Common
    {
    }
    
    /**
     * @implements Common<SuperB>
     */ 
    class B implements Common
    {
    }
    
    interface Super
    {
    }
    
    class SuperA implements Super
    {
        public function callA(): void{}
    }
    
    class SuperB implements Super
    {
        public function callB(): void{}
    }
    

    The factory then needs to have this signature:

    /**
     * @template T of Super
     * @param Common<T> $obj
     * @return T
     */
    public function transform($obj)