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Date: Fri, 29 Apr 2016 18:46:52 +0100
Message-ID: <CAFKaYMqcJtv-oLuyO2riwzN2PNaK21b1toyzLBzxsc082V92Sg@mail.gmail.com>
To: Mathieu Rochette <mathieu@texthtml.net>, Rasmus Schultz <rasmus@mindplay.dk>
Cc: Rowan Collins <rowan.collins@gmail.com>, PHP internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=001a1145e6546057060531a33a97
Subject: Re: [PHP-DEV] [RFC:generics]
From: codekestrel@googlemail.com (Dominic Grostate)

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Content-Type: text/plain; charset=UTF-8

Yes this makes perfect sense now.
On 29 Apr 2016 6:44 p.m., "Mathieu Rochette" <mathieu@texthtml.net> wrote:

>
>
> On 04/29/2016 06:02 PM, Rowan Collins wrote:
> > Hi Dominic,
> >
> > Sorry, I think this may have got sent too early by mistake; please
> > ignore the duplicate!
> >
> > Also, it's crossed in the post with your next message, but I'll send
> > it anyway, in case it helps anyone understand.
> >
> >
> > Dominic Grostate wrote on 29/04/2016 16:45:
> >> It's not insane, it's liskov substitution.  But that is an insane
> >> example, typical of Java to create a type that extends it's parent by
> >> adding value constraints.. directly violating the principle.
> >
> >
> > Let's translate the covariant example code into English:
> >
> > function addPiToList(List<Number> $list): void
> > // "I am a function that accepts any list of Numbers"
> > {
> > $list->add(3.1415);
> > // "To the list I was given, add the Double 3.1415, which is a kind of
> > Number"
> > }
> >
> > $li = new List<Integer>;
> > // "This is a new list of Integers"
> > addPiToList($li)
> > // "You asked for a list of Numbers; Integers are Numbers, so I can
> > give you my list"
> >
> > So far, so reasonable. But now lets see what gets executed:
> >
> > $list->add(3.1415);
> > // "To the list of Integers you gave me, I will add the Double 3.1415"
> > // "Oops! A Double isn't allowed in your list, I will now produce a
> > Type Error"
> >
> >
> >
> > Now the same with the contravariant List<? super Double> example:
> >
> > function addPiToList(List<? super Double> $list): void
> > // "I am a function that accepts a list of any category, as long as
> > that category includes Doubles"
> > {
> > $list->add(3.1415);
> > // "To the list I was given, add the Double 3.1415"
> > }
> >
> > $ln = new List<Number>;
> > // "This is a new list of Numbers, it can mix Integers and Doubles,
> > because they are both types of Number"
> > addPiToList($ln)
> > // "You asked for a list that can contain Doubles; my list can contain
> > any Number, including Doubles, so I can give you my list"
> >
> > And inside the function:
> >
> > $list->add(3.1415);
> > // "To the list of Numbers you gave me, I will add the Double 3.1415"
> > // "Yay, it worked. :)"
> >
> >
> > Contravariance always seems counterintuitive, but it turns out it
> > really is the appropriate constraint in certain situations.
> indeed, thank you for this very detailed explanation.
> can this also be useful/makes sense for declaring a class? (eg: class
> List<? super Double> {}) maybe an abstract class ? or is it just for
> parameters ?
> >
> > Regards,
>
> --
> Mathieu Rochette
>
>

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