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To: Rowan Collins <rowan.collins@gmail.com>,
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Subject: Re: [PHP-DEV] [RFC:generics]
From: mathieu@texthtml.net (Mathieu Rochette)

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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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