Re: [RFC:generics]

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]