Product.scala trait中productPrefix def的实际使用

Question

Maybe silly question, but I can't find any useful explanation about this. I've read the documentation, but I'm confused, maybe one use is to compare types, at some extent. Any idea or practical example?

Blockquote

A string used in the toString methods of derived classes. Implementations may override this method to prepend a string prefix to the result of toString methods. Returns: in the default implementation, the empty string Blockquote

Answer 1

Tuples (which extend Product ) are printed like this:

(x1, ..., xN)
^                  opening parenthesis
 ^                 elements
   ^               commas
            ^      closing parenthesis

Case classes (which also extend Product ) are printed like this:

CaseClassName(x1, ..., xN)
^^^^^^^^^^^^^                    class name
             ^                   opening parenthesis
              ^                  elements
                ^                commas
                         ^       closing parenthesis

Since both inherit from Product , it's possible to provide a default toString implementation once in the Product -trait, which produces something like:

${productPrefix}(x1, ..., xN)   // ascii-art, not actual code

with productPrefix = "" in the case of tuples and productPrefix = "CaseClassName" for case classes.

Given that Tuple s are such an integral part of the language, I could imagine that using the inheritance of a method implementation from the Product trait could have served an actual purpose of "dogfooding" in the earliest phases of the compiler development: this might actually be one of the first and one of the simplest uses of trait s. (It seems somewhat plausible to me, but it's purely speculative; I did not attempt to dig into the commit history.)

Otherwise, there is no deeper meaning in it. It's just a part of the toString implementation. You should never rely on the toString implementation for anything, and you should never use it for any kind of program logic.