Each symbol has four components (or "cells"), each of which references another object:
symbol-name
in section Creating and Interning Symbols.
symbol-value
in
section Accessing Variable Values.
symbol-function
in section Accessing Function Cell Contents.
symbol-plist
in section Property Lists.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In GNU Emacs Lisp, this lookup uses a hashing algorithm and an obarray; see section Creating and Interning Symbols.)
In normal usage, the function cell usually contains a function or
macro, as that is what the Lisp interpreter expects to see there
(see section Evaluation). Keyboard macros (see section Keyboard Macros),
keymaps (see section Keymaps) and autoload objects (see section Autoloading) are
also sometimes stored in the function cell of symbols. We often refer
to "the function foo
" when we really mean the function stored
in the function cell of the symbol foo
. We make the distinction
only when necessary.
The property list cell normally should hold a correctly formatted property list (see section Property Lists), as a number of functions expect to see a property list there.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol void
, nor the same as holding the
symbol nil
.) Examining a cell that is void results in an error,
such as `Symbol's value as variable is void'.
The four functions symbol-name
, symbol-value
,
symbol-plist
, and symbol-function
return the contents of
the four cells of a symbol. Here as an example we show the contents of
the four cells of the symbol buffer-file-name
:
(symbol-name 'buffer-file-name) => "buffer-file-name" (symbol-value 'buffer-file-name) => "/gnu/elisp/symbols.texi" (symbol-plist 'buffer-file-name) => (variable-documentation 29529) (symbol-function 'buffer-file-name) => #<subr buffer-file-name>
Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the Emacs Lisp Manual.
The property list cell contains the list (variable-documentation
29529)
which tells the documentation functions where to find the
documentation string for the variable buffer-file-name
in the
`DOC' file. (29529 is the offset from the beginning of the
`DOC' file to where that documentation string begins.) The
function cell contains the function for returning the name of the file.
buffer-file-name
names a primitive function, which has no read
syntax and prints in hash notation (see section Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
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