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The Ruby Programming Language by David Flanagan, Yukihiro Matsumoto

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Symbols

A typical implementation of a Ruby interpreter maintains a symbol table in which it stores the names of all the classes, methods, and variables it knows about. This allows such an interpreter to avoid most string comparisons: it refers to method names (for example) by their position in this symbol table. This turns a relatively expensive string operation into a relatively cheap integer operation.

These symbols are not purely internal to the interpreter; they can also be used by Ruby programs. A Symbol object refers to a symbol. A symbol literal is written by prefixing an identifier or string with a colon:

:symbol                   # A Symbol literal
:"symbol"                 # The same literal
:'another long symbol'    # Quotes are useful for symbols with spaces
s = "string"
sym = :"#{s}"             # The Symbol :string

Symbols also have a %s literal syntax that allows arbitrary delimiters in the same way that %q and %Q can be used for string literals:

%s["]     # Same as :'"'

Symbols are often used to refer to method names in reflective code. For example, suppose we want to know if some object has an each method:

o.respond_to? :each

Here’s another example. It tests whether a given object responds to a specified method, and, if so, invokes that method:

name = :size
if o.respond_to? name
  o.send(name)
end

You can convert a String to a Symbol using the intern or to_sym methods. And you can convert a Symbol back into a String with the to_s method or its alias id2name:

str = "string" # Begin with a string sym = str.intern # Convert ...

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