Add effect hadling support

[etheilred]

Look at the following example:

void A()
        {
            try
            {
                var t = new User("Mark").Get() ?? throw new ArgumentNullException();
                Console.WriteLine(t);
            }
            catch (ArgumentNullException e)
            {
                // Handle exception
            }
        }
    }

Obviously, if Get() returns null, print will not be called. What can be done:

 void A()
        {
            try
            {
                var t = new User("Mark").Get() ?? perform new ArgumentNullEffect();
                Console.WriteLine(t);
            }
            handle (ArgumentNullEffect)
            {
                resume Users.Default.Get();
            }
        }

perform / handle idiom allows to define a common way of recovering from an error and continue execution. This is more handy than if ... else approach because you

1. Write handler's code once
2. Don't care about errors explicitly in code - it is done in the outer block

[spydacarnage]

That, to me, looks far too much like On Error Goto ...
But in that example, why not just write:

void A()
{
    var t = new User("Mark").Get();
    if (t != null)
        Console.WriteLine(t);
}

[svick]

I don't understand how would this feature be actually useful, but that might be just because of the example. Because in that example:

1. new User("Mark").Get() ?? Users.Default.Get() would be clearer and shorter.
2. Even if you were repeatedly calling new User(name).Get() where you wanted to substitute Default on failure, I think writing a helper function like GetUserOrDefault() would be a better solution.
3. There is seemingly no connection between ArgumentNullEffect and Users, so that code looks brittle to me: if another part of the code "performed" ArgumentNullEffect and you would "resume" it with Users.Default.Get(), that would be a bug.

Maybe you have a more convincing explanation and example, showing why this feature would be useful?