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Date: Wed, 20 Feb 2013 10:07:59 +0100
To: Stas Malyshev <smalyshev@sugarcrm.com>
Cc: Derick Rethans <derick@php.net>, <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Questions regarding DateTimeImmutable
From: glopes@nebm.ist.utl.pt (Gustavo Lopes)

Em 2013-02-20 9:41, Stas Malyshev escreveu:
> There's nothing that "everybody" is
> doing - there are tons of different usage patterns, some of them 
> common,
> some incompatible. We need to look on what is more probable - that
> people use DateTime as hint for operations that do not modify the
> incoming object or that they do modify it and also would put
> DateTimeImmutable in this code.

No, we do not need to look on what is more probable. It's only when we 
select this very ill-advised option, that we need to start guessing if 
people used all the guarantees of DateTime (namely its side-effects) or 
not.

> So on one side we have existing code that would not accept
> DateTimeImmutable without extensive modification,

To "accept a DateTimeImmutable" is not the same as "no recoverable 
fatal error when a DateTimeImmutable is passed". You can't possibly know 
whether passing a DateTimeImmutable is safe without reviewing the code 
on all the call hierarchy starting from where your DateTimeImmutable is 
passed.

> unless we create the
> inheritance, on the other side we have some code that modifies 
> DateTime
> and should be protected from passing DateTimeImmutable - but there's 
> no
> such code yet since nobody passes DateTimeImmutable anywhere yet, so 
> we
> don't make any problem for existing code.

"[C]ode that modifies DateTime and should be protected from passing 
DateTimeImmutable" DOES exist already. Saying that it's not a problem 
for existing code because no one passes DateTimeImmutable is a 
misleading statement -- it assumes codebases exist in isolation. But 
current libraries that take DateTime will still be used in 5.5; in fact, 
that's the basis of your whole argument for this choice.

> I find it highly likely that majoirty of DateTime users don't care if
> it's mutable or not since they either don't change it (most frequent 
> use
> case) or use something like $foo = $bar->modify("+1 day"); and never
> rely on the fact that $bar was modified. Of course, I have no data 
> about
> how many but I find it highly likely that many users simply don't 
> care
> about the difference.

Here is some data:

http://code.google.com/codesearch#search/&q=%5C$date-%3Emodify%20lang:php&type=cs&sq=

As you can see, your assumption is false. In the majority of cases, 
there is no assignment.

-- 
Gustavo Lopes