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Date: Fri, 29 Apr 2016 16:55:39 +0100
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Subject: Re: [PHP-DEV] [RFC:generics]
From: rowan.collins@gmail.com (Rowan Collins)

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.


Read the example again, and translate it 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 go through the same with the amended List<? super Double> example:

function addPiToList(List<? super Double> $list): void
// "I am a function that accepts any list, whose members can include Double"
{
$list->add(3.1415);
// "To the list I was given, add the Double 3.1415"
}

$ln = new List<Number>;
// "This is a new list of Numbers, it can mix Integers and Doubles"
addPiToList($ln)
// "You asked for a list that can contain Integers; 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"