Psr7 Http Message, why immutable?

I am looking at the PSR-7 interfaces and thinking of a way on how to implement them.

I've also been reading this blog post. Apparently the objects that implement the PSR-7 interfaces must be immutable.

So if I implement the withProtocolVersion method from MessageInterface then it would look something like this:

public function withProtocolVersion($version)
{
    if ( $this->protocol === $version )
    {
        return $this;
    }

    $new = clone $this;
    $new->protocol = $version;
    return $new;
}

My question really is, why immutable? Why not simply do a return $this;?

It's not that I'm concerned about the amount of memory it allocates, I just really do not see any benefit in keeping it immutable.

Like the blog posts says, when you do this:

$request = $request
    ->withMethod('POST')
    ->withUrl(new Url('http://example.org/')
    ->withHeader('Content-Type', 'text/plain');

Then four copies are created, but the end result in $request is the same as when I would simply use return $this, right?

Why is the decision made to keep it immutable. So why do I have to do a clone $this? What's the benefit of it?

I'm not really getting the idea begind it.

Solution

I suggest you to read this document, where all the design choices are explained in detail.

In particular you should read the Why value objects? and the New instances vs returning $this sections.

The key points are the following:

In essence, modeling HTTP messages as value objects ensures the integrity of the message state, and prevents the need for bi-directional dependencies, which can often go out-of-sync or lead to debugging or performance issues.

and

These operations can be accomplished with value objects as well, with a number of benefits:

The original request state can be stored for retrieval by any consumer.

A default response state can be created with default headers and/or message body.

If you want to dig deeper, I'd recommend to look in the history of the fig mailing list (you can find it here), where there was a lot of discussion regarding the immutability of the objects