Add null-aware subscript operator ?[] to Language Tour

[mleonhard]

The page https://dart.dev/guides/language/language-tour#other-operators
lists operators (), [], ., and ?.. It is missing ?[]. I wasted many minutes looking for it and finally found it in dart-lang/language#376 . Let's add it to the language tour to save other users' time.

[mmcdon20]

I agree that ?[] should be mentioned in the language tour.

I also wish the language tour would mention that async, async*, and sync* keywords can be used with anonymous functions.

[parlough]

@kwalrath I noticed we currently call the [] operator the "List access" operator here, do we want to name that something more generic like the "subscript operator"? I don't believe the spec has a name for it as technically that is dependent on each individual overriding case, but it seems subscripting is a commonly accepted name and is also what Swift uses.

[kwalrath]

I was going to say that we should use the name in the spec, but it seems to studiously avoid naming [] (if it ever did name it). We might just want to call it the [] operator, like the spec does. Edit: I see that it's helpful to have it in the table. I could ask Leaf or Bob about what they'd call it.

Btw, ?[ appears to be in the spec only once, in a table ("Figure 3") in section 17.20/17.21 (p. 148) that lists ways to invoke methods, along with each way's desugared form.

[kwalrath]

@munificent or @leafpetersen, does subscript operator work for you, or do you have another suggestion? You can look at #3674 (an update to https://dart.dev/guides/language/language-tour#other-operators) to see how it'd be used.

[munificent]

Seems like languages are fairly evenly split between "subscript" and "index":

• Python calls it subscription. So do Swift and C++.

• C# calls them indexers. Kotlin calls it indexed access. I'm not sure how the language itself refers to it, but Programming Ruby calls it the indexing operator.

I personally lean towards "subscript" because "index" is used for other things like numeric loop variables.

So, for completeness, I'd use:

• [] subscript operator
• []= subscript setter operator
• ?[] null-aware subscript operator
• ?[]= null-aware subscript setter operator