A corollary to this is when to use top level classes vs. nested classes.

do you mean top level vs nested classes, or nested classes vs inner classes?

Yes, nested. Corrected.

Responding to the main question: • Use top-level functions by default. • Use an inner function if it is: ◦ very short ◦ only makes sense when used by its enclosing function ◦ is used more than once in the enclosing function • Use a lambda when it is: ◦ short ◦ only makes sense when used by its enclosing function ◦ is only used once You can interpolate this principle to nested classes and anonymous classes.

@Klitos Kyriacou are those your own conversations? Have you seen those conventions published anywhere?

I haven't seen them published, but I was reporting my observations based on all the Kotlin code I've seen over the past couple of years and seeing what looks readable/unreadable and also seeing what the common idioms seem to be.