Files

The unit of work for the compiler is a “compilation unit”, which is usually just an ordinary file. The Scala language places few restrictions on how code is organized across files. The definition of a class, or equivalently a trait or object, can’t be split over multiple files, so it must be contained within a single file. A class and its companion object must be defined together in the same file.

[todesking]

Scala、本家のコーディング規約では1ファイルに複数置いていい条件とファイル命名規則について書いてあります