Is there a valid reason to try casting a List to a...

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[jbnizet]

Is there a valid reason to try casting a List to a MutableList? If I choose to return a List from a method, isn't it clear that the list is not mutable and shouldn't be modified? Regarding the platform types, couldn't we just define a wrapping factory function: fun <T> unmodifiableList(list: List<T>): List<T> = Collections.unmodifiableList(list)? Note that I(m not saying Jayson's immutable collections are a bad idea. I'm just trying to understand the differences, pros and cons of the approaches.