5. Fan types are not-nullable by default, with the concept built into the core of the language, instead of being "Option"al.
This particular subject is interesting. I've rarely seen ClassCastExceptions in practice, but I've seens plenty of NullPointerExceptions. Without good documentation (remember path of least resistance here), it's really hard to know whether a method expects to support null pointers or not. And in a method that has been around for months or years, it's hard to analyze whether all the callers (however far up the call chain) are passing in nulls or not. It's common to get null pointer exceptions in random places in your code all the time with enough developers and a large enough code base. And the solution to random breakage is ad hoc null checks and best-guess alternative behavior. That clutters up code and leaves expectations still unclear.
So, my answer is that nulls are usually evil. Apparently, I'm not alone in this opinion. Tony Hoare, for example, called null references his "billion dollar mistake" in ALGOL W.
My opinion of the right answer for Java is "don't use null if you can avoid it" and "document your methods as whether they support null" and "throw exceptions early if you get a null when you shouldn't". That takes a lot of manual effort in Java. And then there's the primitives not-nullable vs. objects nullable distinction, which is there mostly for convenience/performance. But it leaves the language more complicated than necessary.
So, both Fan and Scala seem to feel nulls are bad, too. They both discourage them. Fan avoids them by making every type not-nullable by default:
Str a := "hello"
Str? b := "world"
b = null // okay
a = b // NullErr run-time error
Str c := null // compile-time error
Lovely, lovely, in my opinion. The same type rules apply for method parameters. Yes, you should still document what your parameters mean, but your path of least resistance leaves you safe from nulls by default. And often, I think that's what coders mean, anyway. Usually, you get NullPointerExceptions because you just presume everything's not null, without even thinking about it. At least, that's how it often seems to me.
Also in Fan, the types 'Bool', 'Float' (64-bit), and 'Int' (64-bit) can be nullable or not. When not nullable, they are called "value types" because they are stored expanded (using 'boolean', 'double', and 'int' primitives in the JVM, for example). Fan also supports autoboxing along these lines. Happily, the '==' operator will compare values for these types (and is one of the overloadable operators for your own types, though I don't recommend overloading except for 'const' types in most cases). Anyway, this strategy also allows for faster math in many cases vs. purely reference-based languages. There are still some improvements needed relative to making the value types blend more transparently into the rest of the type system, but they are in the queue. The final goal is to make it blend almost seamlessly into the rest of the null-vs.-not-null type system.
Scala also recommends against null but in a very different way. They have very distinct reference vs. value types. All value types are predefined (I think) and mostly (but not entirely) correspond to the primitive types in Java. The reference types correspond to objects in Java. All reference types are nullable, but good style says don't use null. At least, when you don't need to interoperate with Java or whatnot. So, just keep in your head not to use null. Instead, you use 'Option' (usually, as there are other alternatives, too). My Fan examples above now look like this in Scala (if I'm not making any mistakes at the moment -- already fixed a few since my original post):
var a: String = "hello"
var b: Option[String] = Some("world")
b = None
a = b.get // NoSuchElementException run-time error
var c: String = None // compile-time error
I'm ignoring 'getOrElse' at the moment (just as I ignored the elvis '?:' operator in Fan above), by the way. I'm just focusing on the basic rules. My opinions on handling nulls when you have them are beyond my current scope.
So again, Fan has a (mostly) unified type system defaulting to not-null. Scala has a branched type system supporting value types on one side and and nullable references on the other, but don't use null. I think Fan's system is simpler, and I like that it pushes not-null into the strong position.
Well, I could be done here, but I'm not.
There's another word in Java that gives the same idea as the English word "null". That word is "void". If you know Java, you know what both keywords mean, but I think the relationship is interesting, and it brings up additional exploration for Fan vs. Scala, too. The value "null" is actually out there. It's a real thing that represents "no object". On the other hand, "void" really means "nothing". It doesn't exist. You can't assign it to anything. As a type, it is an empty set. I should also mention 'Void' in Java as the reflection-friendly type corresponding to the keyword 'void'.
Scala quiz time! Instead of describing what they mean, I'll give a list of similar concepts in Scala and see if you can match them with the correct meaning. If you read above, you'll get some of these. And maybe you can guess the others. First the Scala list of terms (all related to null and void in Java):
And here are the definitions to match them against (in a mixed order):
A. The empty list.
B. The empty set type, corresponding to the meaning of 'void' in Java. The subtype of all value and reference types in Scala. The type parameter used for the empty list.
C. The single instance of type 'Unit'. I think it's purposely not supposed to mean anything.
D. The single instance of type 'Null'. Corresponds to 'null' in Java, but don't use it that way.
E. A subtype of all reference types in Scala whose only value is 'null'.
F. The instance of type 'Option' that means the option was not chose to be 'Some' value. Use with 'Option' types instead of 'null' for direct reference use. Also a kind of empty list.
G. A value type with only one (meaningless) member. Used in place of 'void', although it has a different meaning in my opinion.
Fan quiz time! I'm leaving out empty lists here, although you can have those in Fan just like in Java. They just don't have names or meanings so intertwined with 'null' and 'void', so just like Java, I think they are less relevant in Fan. Anyway, here are the terms I think should be considered:
And here are the definitions to consider:
A. A type that corresponds quite closely to 'void' in Java. An empty set with no members, and only used for return types from functions.
B. A type with just the 'null' reference as a member. Somewhat related to the 'Unit' type in Scala, but I'm not sure it has any practical use. I think I'm glad it's there (just for consistency), and maybe it has some use I haven't figured out yet.
C. Corresponds to 'null' in Java. Assignable to vars of any nullable type.
Anyway, I hope this post was sufficiently readable. I think it's an important subject, and I personally like the solution that I think is simpler, more orthogonal, and less null-friendly. (And that particular personal preference is presumably easy to guess at this point.)