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Date: Fri, 29 Apr 2016 17:02:14 +0100
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Subject: Re: [PHP-DEV] [RFC:generics]
From: rowan.collins@gmail.com (Rowan Collins)

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.

Regards,
-- 
Rowan Collins
[IMSoP]