Feature generics

[dellisd]

The class definition for features could be modified to include a generic type parameter to specify what type of geometry a feature holds. e.g.

class Feature<out T : Geometry?>(
    val geometry: T
// Rest of Feature class definition...

val p: Feature<Point> = Feature(Point(...))
p.geometry // is Point

This would facilitate passing Features in to turf functions as noted in #71.

Pros

• Feature geometry can be made type-safe, reducing the number of runtime type checks on a feature's geometry in some cases
• The Turf port can take advantage of this allowing for only certain types of features to be passed into some functions

Cons

• Collections of features often hold many different types of geometry so any type-safety gains from this feature would be lost, and either star-projection or filling the generic type with Geometry or Geometry? repeatedly would be required.
• Some Turf function signatures will conflict for specialized feature overloads. e.g. length(feature: Feature<?>, ...)

Open Questions

• Should FeatureCollections have a generic type applied to them too?

FeatureCollection<LineString> // denotes a collection of LineString features

• Should the properties of the feature class also be made a generic type to allow for type-safe feature properties?

[elcolto]

Regarding the cons, is it despite to use a generic type parameter? Then I'd like to open a PR for it. I think a FeatureCollection shouldn't have a generic type parameter. Cause the features could be polymorph so in many cases it would be FeatureCollection<Geometry>, which may be some overhead to define it every time. Even on updating/edit a collection this type parameter may have to be updated, this is unhandy on consumer side.