Agreed on all counts . Not sure what the C# team's desire is for another classification mechanism (record types), but it could be seen as an opportunity to fix some of the problems of the past (mutable types, nullable object references).

Perhaps C#'s record type could enforce immutability on not just the record but anything it's assigned to (maybe by using 'let'). That would allow the old mutable way to co-exist with language enforced immutability. It doesn't seem like it would clutter up or modify any existing code either, so if you don't like it, you never need to see it.

Not sure how that would work when assigning to a class's member property/field. Perhaps you opt out of immutability (of the reference) with a 'mutable' keyword:

    class AClass
    {
        public mutable ARecordType Value;
    }

Functional languages also 'extend' the behaviour of a type by defining functions that work with the type. But with an object-oriented language like C# you'd still expect the functionality to come with the type I think. So I suspect it would have to look something like this:

    type Vector
    {
        float X;
        float Y;
        float Z;

        int Length => Math.Sqrt(X*X+Y*Y+Z*Z);

        Vector Add( Vector value ) => new Vector { X+value.X,Y+value.Y,Z+value.Z };
    }

So it turns the entire class into one big constructor (with everything public by default). All non-expression fields should be provided when you instantiate the object:

    let obj = new Vector { X=1, Y=2, X=3 };

I'd be very happy with that. Maybe I'll fork the compiler and have a go... if I can just find the time.... gah