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To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.
When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it "hashes" those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see 7. Hash Tables.)
If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.
Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.
Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the M-x command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name.
No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.
Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs.
In Emacs Lisp, an obarray is actually a vector. Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty. Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket. Because these links are invisible, there is no way to
find all the symbols in an obarray except using mapatoms (below).
The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, so you can create an obarray
with (make-vector length 0). This is the only
valid way to create an obarray. Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.
Do not try to put symbols in an obarray yourself. This does
not work--only intern can enter a symbol in an obarray properly.
Common Lisp note: In Common Lisp, a single symbol may be interned in several obarrays.
Most of the functions below take a name and sometimes an obarray as
arguments. A wrong-type-argument error is signaled if the name
is not a string, or if the obarray is not a vector.
(symbol-name 'foo)
=> "foo"
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Warning: Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don't do it!
nil. In the example below,
the value of sym is not eq to foo because it is a
distinct uninterned symbol whose name is also `foo'.
(setq sym (make-symbol "foo"))
=> foo
(eq sym 'foo)
=> nil
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intern
creates a new one, adds it to the obarray, and returns it. If
obarray is omitted, the value of the global variable
obarray is used.
(setq sym (intern "foo"))
=> foo
(eq sym 'foo)
=> t
(setq sym1 (intern "foo" other-obarray))
=> foo
(eq sym1 'foo)
=> nil
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Common Lisp note: In Common Lisp, you can intern an existing symbol
in an obarray. In Emacs Lisp, you cannot do this, because the argument
to intern must be a string, not a symbol.
nil if obarray has no symbol with that name.
Therefore, you can use intern-soft to test whether a symbol with
a given name is already interned. If obarray is omitted, the
value of the global variable obarray is used.
The argument name may also be a symbol; in that case,
the function returns name if name is interned
in the specified obarray, and otherwise nil.
(intern-soft "frazzle") ; No such symbol exists.
=> nil
(make-symbol "frazzle") ; Create an uninterned one.
=> frazzle
(intern-soft "frazzle") ; That one cannot be found.
=> nil
(setq sym (intern "frazzle")) ; Create an interned one.
=> frazzle
(intern-soft "frazzle") ; That one can be found!
=> frazzle
(eq sym 'frazzle) ; And it is the same one.
=> t
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intern and
read.
nil. If obarray is
omitted, it defaults to the value of obarray, the standard
obarray for ordinary symbols.
(setq count 0)
=> 0
(defun count-syms (s)
(setq count (1+ count)))
=> count-syms
(mapatoms 'count-syms)
=> nil
count
=> 1871
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See documentation in 24.2 Access to Documentation Strings, for another
example using mapatoms.
symbol is not actually in the obarray, unintern does
nothing. If obarray is nil, the current obarray is used.
If you provide a string instead of a symbol as symbol, it stands
for a symbol name. Then unintern deletes the symbol (if any) in
the obarray which has that name. If there is no such symbol,
unintern does nothing.
If unintern does delete a symbol, it returns t. Otherwise
it returns nil.
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