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Date: Mon, 9 Mar 2015 09:17:58 -0500
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To: Derick Rethans <derick@php.net>
Cc: Trevor Suarez <rican7@gmail.com>, PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [PR] DateTime::createFromImmutable() method
From: leverton@gmail.com (Matthew Leverton)

On Mon, Mar 9, 2015 at 5:35 AM, Derick Rethans <derick@php.net> wrote:
> On Thu, 5 Mar 2015, Trevor Suarez wrote:
>
>> Good morning internals!
>>
>> I would like to propose a small addition be made to the DateTime class.
>>
>> https://github.com/php/php-src/pull/1145
>
> When the factory method was added, we had this same discussion. And the
> discussions lead to the conclusion that it does not make sense to have
> this "createFromImmutable" factory method on DateTime. Basically with
> the same reasons that people should type hint against the Interface
> instead.
>
If you have a method that needs to modify the DateTime, then you must
choose between DateTime or DateTimeImmutable. (The interface is read
only.) That is, I think it is poor practice to repeat the following
over and over in every method:

function changeTheDate(DateTimeInterface $dt)
{
  $immutable = new DateTimeImmutable($dt->format('c'));
  $immutable->setTimeZone(...); // etc
  return $someCalculation;
}

That function really needs a DateTimeImmutable, so the burden of
converting it ought to be on the caller. It might be able to optimize
the number of conversions, etc.

I almost always use DateTimeImmutables ... fewer side effects, etc.
However, many libraries and code I interface with require a DateTime
for such things. (Yes, perhaps they just incorrectly type hint for a
DateTime, but I cannot help that.) So I need to convert from
DateTimeImmutable to DateTime.

e.g.,

function foo(DateTime $dt) { }

$dt = new DateTimeImmutable;

foo(new DateTime($dt->format('c')); // the method I use

foo(new DateTime($dt)); // doesn't work

foo(DateTime::fromInstance($dt)); // doesn't exist

(It's possible that
DateTimeImmutable::fromInstance($dateTimeImmutable) could just return
the same object.)

So while not a pressing issue, I do think a more straightforward way
of converting would be good because the reality is that you're going
to need to do the conversions at some point if you interface with
other people's code, no matter how hard you try to use good practices.

--
Matthew Leverton