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| Ruby user's guide | Variables |

Ruby has three kinds of variables, one kind of constant and exactly two pseudo-variables. The variables and the constants have no type. While untyped variables have some drawbacks, they have many more advantages and fit well with ruby's quick and easy philosophy.

Variables must be declared in most languages in order to specify their type, modifiability (i.e., whether they are constants), and scope; since type is not an issue, and the rest is evident from the variable name as you are about to see, we do not need variable declarations in ruby.

The first character of an identifier categorizes it at a glance:

`$` | global variable
`@` | instance variable
`[a-z]` or `_` | local variable
`[A-Z]` | constant

The only exceptions to the above are ruby's pseudo-variables: `self`, which always refers to the currently executing object, and `nil`, which is the meaningless value assigned to uninitialized variables. Both are named as if they are local variables, but `self` is a global variable maintained by the interpreter, and `nil` is really a constant. As these are the only two exceptions, they don't confuse things too much.

You man not assign values to `self` or `nil`. `main`, as a value of `self`, refers to the top-level object:

    ruby> self
       main
    ruby> nil
       nil

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