[mkehrt]

This line of reasoning is utterly alien to me. If your types add mental overhead, you’re doing something wrong. You should be able to rely on the typechecker to check invariants for you, decreasing mental overhead. You have to think about types anyway, especially in an untyped language, where you can’t rely on the typechecker.

[infamouscow]

The leap from thinking about your program with inheritance to thinking about your program with traits is large and cannot be overstated.

It's one of the reasons why Python developers don't like working in Ruby, for example.