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C# and Java optionals aren't very useful, since they would be reference types themselves, and thus could be null!

I would submit Rust and Swift as "decent" type systems, if Haskell is the standard for "good". They do nullability correctly, but lack HKTs.

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C# has non-nullable value types via the `struct` keyword. And C# in fact already has a defined optional type to handle this; it's called `Nullable`. [0]

[0] https://msdn.microsoft.com/en-us/library/b3h38hb0%28v=vs.110...

In Scala, Option[Whatever] may also be null. The difference is that it's considered "some Java compatibility leftover" and never exploited by any sane piece of code.

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