Safe navigation operator

[SomeRanDev]

Add safe access operators for nullable values.

@:nullSafety(Strict)
function add12(myArray: Null<Array<Int>>): Null<Int> {
    myArray?.push(12);
    return myArray?.length;
}

Rendered version

[Aurel300]

I do think this would make for a good addition, also combined with null coalescing (PR #85). For the index and call variants, ?.[ (resp. ?.() might be easier to parse.

[Simn]

The detailed design fails to design how exactly this would be implemented.

Also, there's still the open problem that cond?.1:.2 is valid syntax already.

[Aurel300]

Yes, the detailed design should mention the AST changes at least. I guess the least intrusive change would be an optional flag nullSafe on EField, ECall, and EArray (false by default)?

Also is the cond?.1:.2 really a problem? ?. followed by a letter or underscore begins a null-safe field access, whereas ?. followed by a number is the start of a ternary and a shorthand float literal.

[Simn]

Also is the cond?.1:.2 really a problem? ?. followed by a letter or underscore begins a null-safe field access, wherease ?. followed by a number is the start of a ternary and a shorthand float literal.

Perhaps, but this is still something that has to be addressed in the proposal.

Anyway, my tentative opinion here is that ?. and ?[ are useful, but ?( is a step too far. I'll elaborate a bit later if necessary.

[ibilon]

but ?( is a step too far.

I find it useful for optional callbacks.

[kaikoga]

(nullableDynamicArrayContainsString?[0]:String) is valid Haxe syntax, given var String = [1]; and var nullableDynamicArrayContainsString = false;.

https://try.haxe.org/index.php#f251cdC9

[RblSb]

Maybe instead of ?[ we can add get/set methods to Array after, like in Map, and use arr?.get(0) for such cases?

[Gama11]

@RblSb Then we'll suddenly have code style debates about which variant should be used for array access (like we already have for maps). :/

[SomeRanDev]

(nullableDynamicArrayContainsString?[0]:String) is valid Haxe syntax, given var String = [1]; and var nullableDynamicArrayContainsString = false;.

Wow, in retrospect I'm very embarrassed I overlooked this. The same can also be applied to the safe function call. It now makes a lot more sense to me why JS uses jsFunction?.() or jsArray?.[0]. With Haxe having very similar syntax, I think it makes sense to shoot for that (assuming safe array-access/function call are even included)? I'll update the proposal.

Yes, the detailed design should mention the AST changes at least. I guess the least intrusive change would be an optional flag nullSafe on EField, ECall, and EArray (false by default)?

I'll go ahead and move it from "Impact on Existing Code" to the "Detailed Design" with a bit more specificity. Wasn't sure if optional flags were desired when it came to AST? Figured new EMaybeField/EMaybeCall/etc would be used, or an ESafeNav(e:Expr) that wraps around EField, ECall, or EArray, but I have no experience with designing Haxe AST, so I can't say myself.

Also, there's still the open problem that cond?.1:.2 is valid syntax already.

Will acknowledge in proposal as well.

[grepsuzette]

It is adding syntax for something which, as a whole, is like quicksands.

In the end it may encourage some people using null over things like haxe.ds.Option. While I occasionnally like to use nullability I am in general not for creating those syntaxes.

[back2dos]

1. Option generally yields poorer performance (more work for the GC, slower access).
2. Code is not written in a vacuum. When you load 3rd party data, there can be whole object trees that are nullable. Time is better spent on helping people with common situations beyond their control, than trying to police them into using one thing vs. another, in the hopes of preventing them from doing stupid things (which is a lost cause).
3. When using null safety, there's nothing wrong with using Null. One could define map / flatMap / or in userland (as is common practice for Option). This is no different, except that it's less noisy, and it is built right into the language core - as is null itself.

[grepsuzette]

But that's what I dislike, it's like defining a Maybe monad implemented over null. Why the privilege? Can we imagine they are available as well for something like haxe.ds.Option?

[Aurel300]

It's an operator that will be given a default meaning, just like + has a default meaning for numeric types. If you want safe navigation operators for Option too, you can create OptionTools and (probably macro) methods to handle ?. there. Whether you like that Null is given "privilege" is irrelevant given how common it is in Haxe and even more so external libraries.

[grepsuzette]

Ok I see.

[sebthom]

For me the biggest benefit of a safe navigation operator would be to reduce the boilerplate when using @:nullSafety(StrictThreaded) in conjunction with class/instance variables. So we can avoid having to use temporary variables to satisfy null checks.

In the following case:

class Test {
  static var myArray:Null<Array<Int>>;

  static function main() {
    // without @:nullSafety
    myArray.push(1);

    // with @:nullSafety(Strict)
    if(myArray != null) myArray.push(1);

    // with @:nullSafety(StrictThreaded)
    final myArrayRef = myArray;
    if(myArrayRef != null) {
       myArrayRef.push(1);
    }
  }
}

So classVariable?.member() should be expanded to something like:

final classVariableTmp = classVariable;
if(classVariableTmp != null) {
   classVariableTmp.member();
}

when @:nullSafety(StrictThreaded) is enabled.

[Simn]

This proposal has been accepted, see https://haxe.org/blog/evolution-meeting-2021/ for details.