Ask HN: How do you explain closures?

Re:Ask HN: How do you explain closures? by rleisti on Friday November 06, 2009 @11:35AM (#926290)

You could try an example showing what a closure does 'under the hood' by showing an equivalent non-closure example, like:

With Closure:

  foreach (var obj in objects) {
    AddButton("Delete " + obj.ToString(), () => obj.Delete());
  }

Without closure:

  class DeleteHandler {
    public DeleteHandler(TYPE obj) {
      _obj = obj;
    }
    private TYPE obj;
    public void DoDelete() {
      obj.delete();
    }
  }
  ...
  foreach(var obj in objects) {
    var deleteHandler = new DeleteHandler(obj);
    AddButton("Delete " + obj.ToString(), deleteHandler.DoDelete);
  }

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Re:Ask HN: How do you explain closures? by tome on Friday November 06, 2009 @11:57AM (#926359)

I'm not convinced by the utility of your example. What's wrong with:

    foreach (var obj in objects) {
      AddButton("Delete " + obj.ToString(), obj.Delete);
    }

This is even simpler and doesn't use closures at all.

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Re:Ask HN: How do you explain closures? by rleisti on Friday November 06, 2009 @12:10PM (#926388)

Doh!

It would have made more sense if I had some other piece of information to inject in, like if you had a database reference that needed to be passed in to the Delete() method.

Then:

  AddButton("Delete " + obj.ToString(), () => obj.Delete(db));

Vs.

  var d = new DeleteHandler(obj, db);
  AddButton("Delete " + obj.ToString(), d.DoDelete);

EDIT: fixed

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Re:Ask HN: How do you explain closures? by tome on Friday November 06, 2009 @12:46PM (#926473)

Yup, makes more sense now! This is a good example, I think, to give to people who are used to objects being used for everything. Nice, clean, functional syntax.

(And of course you mean "d.DoDelete")

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