clr array covariance?

[gauthier]

Is this not meant to work with cobra?

class Sample
    shared
        def main
            Sample.foo = @[Bar(), Bar()]
            print Sample.foo

        var foo as IFoo*?

interface IFoo
    pass

class Bar
    implements IFoo
    pass

the following c# sample works ok, and c# 4 supports co-contra variance

Code: Select all

using System;
using System.Collections.Generic;
public class Sample{
   public static void Main(){
      foo = new[] {new Bar{}, new Bar{}};
      Console.WriteLine(foo);
   }
   static IEnumerable<IFoo> foo;
}
interface IFoo{}
class Bar : IFoo{}

I'm not sure of the implications for other backends or implementation, but for csharp it seems to be the right thing to do

[Charles]

I haven't had a chance to look into this yet. Will get back to you.

[Charles]

gauthier informs me that this feature is not critical so I have recorded it in ticket:162.

[Charles]

This is now fixed via a patch submitted by hopscc. Thanks.

[hopscc]

My pleasure

Is there any interest/desire to have similar behavior supported in lists ?

[Charles]

I don't think it will work at run-time with the VM. You cannot generally assign List<of Car> to List<of Shape>. Or did you have something else in mind?

[hopscc]

Same as above code but assign from a list rather than from an array

Code: Select all

Sample.foo = [Bar(), Bar()]


Assign a List<of Car> to List<of ICarLike>

[Charles]

Do you mean this or something else?

Code: Select all

// C#
using System;
using System.Collections.Generic;

interface ICar {
   
}

class Car : ICar {

}

class Program {
   
   public static void Main() {
      List<ICar> icars = new List<ICar>();
      List<Car> cars = new List<Car>();
      icars = cars;
      TakeICars(cars);
   }
   
   static void TakeICars(List<ICar> icars) {
   }

}

C# doesn't like it:

list-covar.cs(17,11): error CS0029: Cannot implicitly convert type
'System.Collections.Generic.List<Car>' to 'System.Collections.Generic.List<ICar>'
list-covar.cs(18,3): error CS1502: The best overloaded method match for
'Program.TakeICars(System.Collections.Generic.List<ICar>)' has some invalid arguments
list-covar.cs(18,13): error CS1503: Argument '1': cannot convert from
'System.Collections.Generic.List<Car>' to 'System.Collections.Generic.List<ICar>'

And I don't think .NET would allow, for example, the parameter passing at run-time even if you tried to sneak by with reflection.

If you meant something else, just give a complete code example.

[gauthier]

Hi, Thanks Chuck, I'll check that in latests revision.

about the idea from hopscc, I think that would be usefull to mimic something like that:

Sample.foo = [Bar() to IFoo, Bar() to IFoo]

[Charles]

The other thing that comes up is that sometimes I want a more general list:

shapes = [Circle(), Circle()]
shapes.add(Square())  # compile-time error because shapes is List<of Circle>

# currenty, ugly workaround:
shapes = List<of Shape>()
shapes.add(Circle())
shapes.add(Circle())
shapes.add(Square())

# proposal 1:
shapes = List<of Shape> [Circle(), Circle()]
shapes.add(Square)

# proposal 2:
shapes = [Circle(), Circle()] <of Shape>
shapes.add(Square)

This can come up with dictionaries as well which take 2 type arguments.

Let me know what you think.

Regarding hopscc proposal, it either won't work with .NET at run-time or I don't understand it. Hence my earlier message. Maybe he meant just for list literals, which would be doable. If he meant for any expression of type List<of T>, I don't think there's a real way to make that work.

More discussion is welcome.