Symbols in Lisp

A symbol in Lisp is unique in a namespace (or package in Common Lisp). Symbols can be tested for equality with the function EQ.

Lisp programs can generate new symbols at runtime. When Lisp reads data that contains textual represented symbols, existing symbols are referenced. If a symbol is unknown, the Lisp reader creates a new symbol.

In Common Lisp, symbols have the following attributes: a name, a value, a function, a list of properties and a package.

In Common Lisp it is also possible that a symbol is not interned in a package. Such symbols can be printed, but when read back, a new symbol needs to be created. Since it is not interned, the original symbol can not be retrieved from a package.

In Common Lisp symbols may use any characters, including whitespace, such as spaces and newlines. If a symbol contains a whitespace character, it needs to be written as |this is a symbol|. Symbols can be used as identifiers for any kind of named programming constructs: variables, functions, macros, classes, types, goto tags and more. Symbols can be interned in a package.

Keyword symbols are self-evaluating, and interned in the package named KEYWORD.

Examples

The following is a simple external representation of a Common Lisp symbol:

this-is-a-symbol

Symbols can contain whitespace (and all other characters):

|This is a symbol with whitespace|

In Common Lisp symbols with a leading colon in their printed representations are keyword symbols. These are interned in the keyword package.

:keyword-symbol

A printed representation of a symbol may include a package name. Two colons are written between the name of the package and the name of the symbol.

package-name::symbol-name

Packages can export symbols. Then only one colon is written between the name of the package and the name of the symbol.

package:exported-symbol

Symbols not interned in a package can also be created and have their own notation:

#:uninterned-symbol