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Comment by louthy | Hacker News Reader

> the semantics of pattern matching as an expression can be subtle, especially when paired with nullability

Could you expand upon that? If I can do this:

    var area = shape is Line l      ? 0
             : shape is Rectangle r ? r.Width * r.Height
             : shape is Circle c    ? Math.PI * c.Radius * c.Radius
             : throw new ApplicationException();

How is this, a collection of predicates and expressions, not the same?

    var area = switch(shape)
    {
        case Line l      => 0;
        case Rectangle r => r.Width * r.Height;
        case Circle c    => Math.PI * c.Radius * c.Radius;
        default          => throw new ApplicationException();
    }

I realise you work on Roslyn (and you said semantics, so my example may be a bit off), so I'd love the detail on why the above is problematic.

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Well, let's say Line, Rectangle, and Circle are reference types.

Which case does null match?

If you match the first type, that's somewhat unsatisfying. But if you only match with default, that's also unsatisfying because now we would have a problem with completeness -- if the match doesn't succeed then you have a potentially unassigned variable.

So now every match expression would have to have a default case just to handle null, but you also don't gain the advantages of static matching because everything matches default, so if you do legitimately forget a case you get no warning.

Existing switch statements are much more resilient to these matters simply because they aren't expected to be exhaustive right now. People are used to the fact that they have to deal with unassigned variables or completeness failures.

The match expression, however, I want to be more like ML where you can get strong guarantees on the "irrefutability" of a match.

> Well, let's say Line, Rectangle, and Circle are reference types.

> Which case does null match?

None of them, it should be a case on its own:

    var area = switch(shape)
    {
        case Line l      => 0;
        case Rectangle r => r.Width * r.Height;
        case Circle c    => Math.PI * c.Radius * c.Radius;
        case null        => throw new ArgumentNullException();
    }

That would allow for completeness checking, and if you do want a catch-all then you'd add a default clause.

I have since looked at the proposal for the expression based switch. TBH, I'll just be happy when it's in the language, the syntax is less important to me; but I am not quite sure it needed such a drastic change - it does feel a touch awkward.

> The match expression, however, I want to be more like ML where you can get strong guarantees on the "irrefutability" of a match.

Music to my ears.

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