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Updates to Dart style guide

Posted by Bob Nystrom

Just a quick note that I've updated the Dart style guide on The changes mostly reflect practices we've already adopted internally, but it's good to write them down so we can try to be as consistent as possible. While coding style is an eternal holy war, I think most of us agree that having a single style that the community uses makes it easier for all of us to share, reuse, and extend code.

Use ";" for empty constructors. This is required for const constructors. So, to be consistent, we use this for all empty constructors (which are pretty common thanks to initializing formals).

Place the super() call last in initialization lists. Dart lets you put it anywhere, but it always gets invoked after the other fields are initialized. Placing it last makes that visually consistent.

Prefer type annotating public APIs. Most of us seem to naturally do this anyway, so this just codifies that. You don't *have* to type annotate all over the place in Dart, but it seems like many of us feel you get the most value out of the language by using types at your API boundaries.

Do not type annotate locals. Conversely, we don't see a lot of value in annotating local variables. Methods tend to be pretty short in modern code and the type of a local is almost always obvious from the initializer. Also, the editor will now do type inference of locals variables declared using var, so you still get the auto-complete you know and love.

Don't use "double" as a type annotation. It doesn't mean what you think. Use "num". If it was up to me, I would just get rid of the "double" type completely, but avoiding it is the next best thing.

Don't type annotate initializing formals. "Initializing formals" are the "this." thing you can do with constructor parameters to initialize a field. Since the field already likely has a type annotation, it's redundant to have it here too.

Don't annotate function expressions. Function expressions are usually short and their terseness is what's nice about it. Adding unnecessary type annotations just clutters it up. This seems to be what we do in practice anyway.

Name libraries and source files using lowercase_with_underscores. This ensures that they are valid identifiers, which may come in handy if we move to an identifier-based #import syntax.

Clarify spacing of "is !". Our repo has a mixture of "is! Foo" and "is !Foo". The latter wins by about a factor of two, and is also consistent with the other spacing rules.

Constructor initialization list formatting. Existing code is all over the place here and no style seems to make initialization lists particularly beautiful, but at least picking one style makes them consistent.

Spacing for "{", "}" and named arguments (":" and "="). The style guide didn't say how things like empty maps should appear ("{}" or "{ }"). Now it does.

Not exactly a super exciting development in the world of Dart, but I think little stuff like this can help make Dart code easier to work with, and that in turn makes it easier for our community to grow.


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