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Date: Wed, 17 Feb 2016 08:26:11 -0700
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To: Kevin Gessner <kgessner@etsy.com>
Cc: internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] Traits with interfaces
From: levim@php.net (Levi Morrison)

On Wed, Feb 17, 2016 at 7:25 AM, Kevin Gessner <kgessner@etsy.com> wrote:
> Hello internals team!  I'd like to propose an RFC to allow traits to
> implement interfaces.
>
> I've noticed s pattern in Etsy's code and elsewhere, where a trait provides
> a common implementation of an interface.  Classes that use the trait are
> required to also explicitly declare the interface to benefit.  I propose
> that traits be permitted to declare and implement interfaces.  Classes that
> use such a trait would then implement the interface, as though it were
> declared on the class, without declaring the interface explicitly.

I am unsure on the particular detail of the class automatically
implementing the interface.

I want to add my personal experience with traits: every time I create
a trait it is to implement an interface. Here is a publicly available
[example with OuterIterator](https://github.com/morrisonlevi/Ardent/blob/master/src/Collection/OuterIteratorTrait.php):

trait OuterIteratorTrait {

    abstract function getInnerIterator() : Iterator;

    function current() {
        return $this->getInnerIterator()->current();
    }

    function next(): void {
        $this->getInnerIterator()->next();
    }

    function key() {
        return $this->getInnerIterator()->key();
    }

    function valid(): bool {
        return $this->getInnerIterator()->valid();
    }

    function rewind(): void {
        $this->getInnerIterator()->rewind();
    }
}

This makes it really easy to implement iterators. The class that uses
the trait can simply implement `getInnerIterator` and then modify only
the methods that differ from standard behavior. Here are a few
examples:

  * https://github.com/morrisonlevi/Ardent/blob/master/src/Collection/HashMapIterator.php
  * https://github.com/morrisonlevi/Ardent/blob/master/src/Collection/HashSetIterator.php
  * https://github.com/morrisonlevi/Ardent/blob/master/src/Collection/SortedMapIterator.php
  * https://github.com/morrisonlevi/Ardent/blob/master/src/Collection/SortedSetIterator.php

I can see how it would be nice to allow the trait to officially
declare that it implements some interface (in this case Iterator or
OuterIterator) which would require that the trait has fully
implemented the required methods (or declared them as abstract). This
would be a small improvement but helpful.

I am less certain about the classes which `use` it automatically
inheriting the interfaces. To clarify: I am neither in favor or
against that part. Or at least at this stage, anyway.