Question

scala> object Test
defined module Test

Why is the defined object Test called ‘module’, not companion object, by the scala interpreter ?

Is there a difference between module and companion object or is it just the same with two different names ?

Answer 1

Technically, there is only one such thing, in the language specification it is mostly called ‘module’, but you also find this statement: “The object definition defines a single object (or: module) …” (Scala Language Specification)

Furthermore, you can only speak of a companion object, when it actually accompanies something:

“Generally, a companion module of a class is an object which has the same name as the class and is defined in the same scope and compilation unit. Conversely, the class is called the companion class of the module.” (again think: companion object = companion module)

Being in companion state adds features to the companion class, namely visibility (e.g., the class can see the private fields of the companion module). Same scope and compilation unit means, they need to be defined in the same source file and same package.

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There is an interesting thread on LtU where Scala’s object versus module terminology is discussed. It contains also a link to a paper by Odersky and Zenger if you are intrigued; showing how they particularly looked at the ML language’s module system (OCaml being a major influence on Scala), and how they frame it as various approaches of modular composition (suggesting that module is a more generic concept; traits as mixin modules, …)