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Comment by redwards510 | Hacker News Reader

Can someone give an example of a use case for a Deconstructor?

    var (ret, err) = Foo();
    if (err != null) {
      ...
    }

So Foo() has to return a class which has a destructor? Seems like just returning a Tuple would be better in that case.

Tuple in C# is a class IIRC. Without destructor, you will have to pull out individual members separately. For example for a tuple of form <Int, Int, Int>, you will do something like `var r = c.getFirst(); var g = c.getSecond();`. With deconstructor, you can do the same via `var (r, g, b) = color;`. In other words, you can say that tuples are way to combine data, while deconstructors tell how to explode it. Since these deconstructors are themselves functions, you can do computation each time, and get different results due to side-effects (probably not good idea).

EDIT: I don't know C#, so syntax is definitely not right.

You would just check t.Item2, no need to assign to variables I'm this example. However, the deconstructor syntax is nice, and now tour functions don't all return Tuple with vaguely named members.

Right. Not great example. What I wanted to say was deconstructor is all about creating new bindings with nicer syntax. You could just do that by calling getters / accessing fields, but you have to write more when you doing that.

Well, it allows you to return multiple objects without using an opaque Tuple or creating your own class. Sure, the other options work, this is just a nicer syntax. I mean, I can all of what C# does in C if I want to, but I appreciate the syntactical sugar.

Basically yes. Except, with the new tuple syntax support in C# it's using a value type internally instead of the old Tuple class (I'm pretty sure Tuple's going to be gaining a set of deconstructors to allow it to participate though). And the pure tuple path apparently doesn't use the deconstructors, there's something fancier going on under the hood which is presumably faster.

But yes you can write a deconstructor for anything you like so it can participate in deconstructing assignment. And yes it can have side effects. And that would be VERY BAD.

OK, but that's not what the TFA describes as a deconstructor.

It's linked to providing these tuple types out of a method call.

So instead of having to declare that Foo returns a FooReturn, it can be declared as (say)

    public (int ret, string err) Foo() { return (5, "something"); }

That's not a deconstructor though, which is what the GP was asking about.

EDIT: Looks like there's confusion... Yes, you can deconstruct a Tuple like this, but C# 7 also introduces a specific Deconstructor pattern in which ANY class can be deconstructed as though it were a Tuple.

So ignoring all the confused replies you got from people who didn't read TFA, it looks like Deconstructors allow a concrete class to participate in multiple assignment (aka deconstruction) just like a Tuple. Like with Tuple, you can deconstruct like this:

    var pix = (int R = 255, int G = 255, int B = 255);
    var (r1, g1, b1) = pix;

Except what if you're also working with 3D location data? E.g.:

    var pos = (int X = 100, int Y = 100, int Z = 100);
    var (x1, y1, z1) = pos;

Both pos and pix have the same underlying type of Tuple<int,int,int>, which could lead to confusion. Better to have concrete Pixel and Position classes. You can then give those classes a Deconstruct method and then use them the same way:

    var pix = new Pixel(255, 255, 255);
    var (r1, g1, b1) = pix;
    var pos = new Position(100, 100, 100);
    var (x, y, z) = pos;

var (r, g, b, a) = pixel

Looks a lot like Elixir's pattern matching to extract values from maps.

    someMap = {1, 2}
    {foo, bar} = someMap
    foo # 1
    bar # 2

This is destructuring assignment, as practiced by many languages before. Pretty much anything in the ML family, for example.

I was just going to say.

The other new features seem like very welcome additions but the deconstructor seems a bit contrived and a rare use-case which does not directly solve an existing problem (also makes it easy to confuse desctructor/deconstructor for newbies).

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