Support non-nullable types.

[DartBot]

Admin comment: This is activaly being worked on, see dart-lang/language#110 and https://dart.dev/null-safety

This issue was originally filed by cflew...@gmail.com

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Short version: Null pointers are a really good way to mess up a program at runtime, and I'd like the Dart team to reevaluate whether they're absolutely required.

Slightly longer version: I would say the #­1 cause of issues in my programs (excluding logical errors/requirements errors) are NPEs. Having a language support NPE removal, be it via some clever compiler warning or simply removing null altogether, would be wonderful. I'm personally partial to Scala's method of null removal, but I'm sure PL gurus like yourselves have seen many others.

Dart has a stated goal of avoiding the creation of programs that "are difficult to debug or maintain." NPEs are a huge pain point in this regard. I'd be really happy if the Dart team reevaluated whether they are absolutely required to achieve the other aims.

[DartBot]

This comment was originally written by johnle...@google.com

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Issue #24 has been merged into this issue.

[DartBot]

This comment was originally written by rchandia@google.com

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some input from the author of null references:

http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare

[DartBot]

This comment was originally written by ivan.the.ter...@gmail.com

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I have to agree. From reading through the tutorial, it sounds like Dart is handling null even worse than most languages. (Even numbers might be null?!)

Given that types are optional, it would make sense to allow null when the type is unspecified (var or dynamic return), but if the user (programmer) is explicitly saying that this variable is an int, why should the type checker assume that null is ok?

Null can be useful, but if I am choosing to explicitly declare a type, please let me also explicitly say whether it is allowed to be null. I personally would prefer option types, but given that this language seems to be trying to be similar to C++/Java/Python, perhaps declaring things nullable would be better. For example, if you write "nullable int n = 42", then "n = null" would be allowed.

Another option would be to explicitly annotate variables as not null, like "not_null int n = 42", but IMHO, the only advantage to that would be that it wouldn't break existing code, and given that this is a new language, that's not a very good reason.

http://qconlondon.com/london-2009/presentation/Null+References:+The+Billion+Dollar+Mistake
http://se.ethz.ch/~meyer/publications/hoare/void-safety.pdf
https://secure.wikimedia.org/wikipedia/en/wiki/Nullable

[DartBot]

This comment was originally written by waqn...@gmail.com

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I don't think it is possible to completely forbid null-references in a language that allows dynamic types.

However, on page 73 of the Dart language specification draft, the question is asked "Should we do something with respect to non-nullable types?" and my answer is YES, PLEASE, I'M BEGGING YOU.

[DartBot]

This comment was originally written by jat@google.com

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There was discussion of allowing int? foo to mean that foo might be nullable, and just int foo would mean it wasn't. However, there were some complications, particularly with the idea of types being optional -- ie, removing a non-nullable type annotation could change the behavior of the program.

[DartBot]

This comment was originally written by waqner...@gmail.com

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May I ask what these complications look like?
When would removing a non-nullable type change the behavior of the program, and how exactly would the behavior be different?

[DartBot]

This comment was originally written by leozc.l...@gmail.com

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I would suggest to borrow C# concept which separate value type from object and only object can be null.

If we want value type (like int) be null we can use a constructor and wrap it as a object nullable<int>.

A plain int can be null is usually a disaster for dev..

[DartBot]

This comment was originally written by ladicek@gmail.com

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Nullable types should definitely be added. At least in the expected form where checker would warn you while prod env would let you assign nulls anyway.

[DartBot]

This comment was originally written by jat@google.com

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One example of changing behavior:

int foo = null; // throws NPE
int? foo = null // ok

If you strip off the types, you change the behavior of the program.

[DartBot]

This comment was originally written by clun...@purelunacy.com

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Nullable types need to exist as long as default constraints are identified for the type. Much like C# default(t) with a user pragma like declaration . This would allow for ease of intrinsic types and nullable types based on conditions.

[DartBot]

This comment was originally written by waqn...@gmail.com

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@JaT I don't understand.
Why would a non-nullable
int foo = null;
throw an exception? I thought the type checker only produces compile-time warnings.

A type warning would be perfectly adequate in this case, and not change the behavior of the program if the type is removed. Or am I missing something?

[DartBot]

This comment was originally written by jat@google.com

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So what happens when you dereference a null pointer (that was declared to be non-nullable) at runtime? I don't see how you usefully continue -- the program is broken, and you are better off failing at the point you assign the null pointer to it.

You can do things like require such variables to be initialized and forbid assignment of a null literal, but in general you are not going to be able to tell if the execution of the program would lead to assigning a null to a variable -- it may even reference external things, such as a server or the DOM, that the compiler can't know the behavior of. Even if you require that such functions be declared as "Foo?" and require explicit null checks when assigning to a "Foo", the server might still return a null when it was declared not to.

[dgrove]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language labels.

[DartBot]

This comment was originally written by drfibonacci@google.com

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Added Triaged label.

[DartBot]

This comment was originally written by ivan.the.ter...@gmail.com

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jat: I thought I read that when running in production mode any type errors were supposed to be ignored, so types don't affect the behavior of the program. When running with checking enabled they do, but that is regardless of whether you allow non-nullable types.

For example:
int foo = 'World'; // throws something? (not in production at least)
var foo = 'World';

The same applies to your argument wrt a server or the DOM. If it can return a null when it was declared not to, couldn't it just as easily return a string when it was declared not to?

If you are really worried that making types non-nullable by default would result in question marks all over the place, could you at least support explicitly marking a variable as non-nullable? That would be completely backwards compatible, but having it sooner would mean library writers would be more likely to use it, which would benefit everyone.

I still think explicitly marking nullable ones is better, because that is when you should be thinking about the fact that it might be null, and the presence of a marker tends to be a better reminder for people than it's absence.