Re: [RFC:generics]

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 > >