Practical examples of using symbols in Scala?
Scala has symbols - names that start with a single quote ' and which are a kind of string constants.
I know symbols from Ruby (where they start with a colon). In Ruby they are used for some meta-programming tasks, like generating getters and setters for member variables (for example attr_reader :name to generate a getter for name).
I haven't seen a lot of use of symbols in Scala code yet. What are practical uses for symbols in Scala?
Answer
Do symbols really fit into Scala?
In the wonderful land of Lisp, code is represented as nested lists of literal objects that denote themselves (strings, numbers, and so on), and symbols, which are used as identifiers for things like classes, functions, and variables. As Lisp code has a very simple structure, Lisp allows the programmer to manipulate it (both at compile-time and run-time). Clearly, when doing this, the programmer will inevitably encounter symbols as data objects.
So symbols are (and need to be) objects in Lisp in any case, so why not use them as hash table keys or as enums as well? It's the natural way of doing things, and it keeps the language simple, as you don't have to define a special enumeration type.
To summarise, symbols are naturally used for code manipulation, enumeration, and keying. But Java people don't use identity as the equivalence relation between hash keys by default (which your traditional Lisp does), so they can just use strings as their keys. Enum types are defined separately in Scala. And finally, code as data isn't supported by the language at all.
So no, my impression is that symbols don't belong in the Scala language. That said, I'm going to keep an eye on the replies to this question. They might still demonstrate a genuine use of symbols in Scala that I can't think of right now.
(Addendum: Depending on the Lisp dialect, Lisp symbols may also be namespace-qualified, which is, of course, an immensely useful feature when manipulating code, and a feature that strings don't have.)