Is a symbol table in Ruby any different from a symbol table in other languages

Question

The wikipedia entry on Symbol tables is a good reference:

http://en.wikipedia.org/wiki/Symbol_table

But as I try to understand symbols in Ruby and how they are represented in the Array of Symbols (returned by the Symbol.all_symbols method),

I'm wondering whether Ruby's approach to the symbol table has any important differences from other languages?

Answer 1

Ruby doesn't really have a "symbol table" in that sense. It has bindings, and symbols (what lispers call atoms) but it isn't really doing it the way that article describes.

So in answer to your question: it isn't so much that ruby has the same thing done differently, but rather that it does two different things (:xxx notation --> unique ids and bindings in scopes) and uses similar / overlapping terminology for them.

To clarify:

The article you link to gives the conventional definition of a symbol table, to wit

where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and sometimes its location

But this isn't what ruby's symbol table does. It just provides a globally unique identity for a certain class of objects which can be written as :something in the source code, including things like :+ and :"Hi bob!" which aren't identifiers. Also, merely using an identifier will not create a corresponding symbol. And finally, none of the information listed in the passage above is stored in ruby's list of symbols.

It's a coincidence of naming, and reading that article will not help you understand ruby's symbols.

Answer 2

The biggest difference is that (like Lisp) Ruby actually has a syntax for symbols, and it's easy to add/remove things at runtime yourself. If you say :balloon (or "balloon".intern) it will intern that for you. Even though you're referring to it by name in your source, internally it's just a pointer in the symbol table. If you compare symbols, it's just a pointer-compare, not a string-compare.

Languages like C don't really have a way to say simply "create a new symbol for me" at runtime. You can do it implicitly at compile-time by defining a function, but that's really its only use. Since C has no syntax for symbols, if you want to be able to say Balloon in your program but be able to compare it with a single machine instruction, you use enums (or #defines).

In Ruby, it takes only one character to make a symbol, so you can use it for all kinds of things (like hash keys).

Answer 3

Symbols in Ruby are used where other languages tend to use enums, defines, constants and the like. They're also often used for associative keys. Their use has little to do with a symbol table as discussed in that article, except that they obviously exist in one.