Restore support for static interface properties

[jfirebaugh]

Support for static properties on interfaces was removed in 0.62.0 because they were "never well supported, and in most cases their types were not actually checked".

Was there also a type theoretical basis for removing them, or was it merely pragmatic, as the changlog suggests? If the latter, I would like to request that they be well supported, and their types actually checked. mapbox-gl-js was relying in several places on them, and when upgrading to 0.62.0, it appears I will not only have to comment out or remove these properties, but also disable type checking with any casts where the properties are used.

[samwgoldman]

Where are they used in mapbox-gl-js? Happy to take a look and recommend alternatives.

[jfirebaugh]

Thanks @samwgoldman! The use that looks like it's going to require an any cast is this one. Expression is an interface for an AST node type, parse implementations define recursive-descent parsing for concrete expression types. parse is used here, where Expr is of type Class<Expression>.

[samwgoldman]

Instead of a single Expression interface with statics and non-statics, what if you exported two interfaces, Expression and ExpressionStatics, and used ExpressionStatics instead of Class<Expression>?

[samwgoldman]

Going to close this, but let me know if you run into any further issues.

[akalin]

I have a similar problem: I define a Field interface with static Zero and One members here: https://github.com/akalin/intro-erasure-codes/blob/master/field.js#L15 , and I use it in a Matrix class here: https://github.com/akalin/intro-erasure-codes/blob/234257f63545d59f078738428543337809cb46b4/matrix.js#L60

Splitting Field into Field and FieldStatics doesn't help, since I'd still have to convert a Class<T> into a FieldStatics<T>.

The best thing I can think of is to replace Zero and One with zero-arity functions zero() and one(), which isn't great since I'd have to keep around an instance of T instead of just the constructor function, but I was wondering if you know of other alternatives.

[samwgoldman]

@akalin Not sure what's best for you, but would another option be to add zero: T, one: T as parameters to the Matrix constructor?

[akalin]

Yeah, that's another option. Pretty wordy, though. Thanks anyway!

[rhengles]

Static interface properties are used in Vue.js:

https://github.com/vuejs/vue/blob/dev/flow/component.js#L7