ensures this: every time it reads a symbol, it looks for an existing
symbol with the specified name before it creates a new one. To get a
symbol's name, use the function @code{symbol-name} (@pxref{Creating
-Symbols}). Nonwithstanding each symbol having only one unique _print
-name_, it is nevertheless possible to refer to that same symbol via
-different terms called ``shorthands'' (@pxref{Shorthands}).
+Symbols}). However, although each symbol has only one unique
+@emph{print name}, it is nevertheless possible to refer to that same
+symbol via different alias names called ``shorthands''
+(@pxref{Shorthands}).
The value cell holds a symbol's value as a variable, which is what
you get if the symbol itself is evaluated as a Lisp expression.
symbol every time it reads the same sequence of characters in the same
context. Failure to do so would cause complete confusion.
- When the Lisp reader encounters a reference to symbol in the source
-code, it reads all the characters of that name. Then it looks up that
-name in a table called an @dfn{obarray} to find the symbol that the
-programmer meant.
-
@cindex symbol name hashing
@cindex hashing
@cindex obarray
@cindex bucket (in obarray)
-One the techniques used in this lookup is called hashing, 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 @dfn{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 @ref{Hash Tables}.)
-
-@cindex shorthands
-@cindex namespacing
-@cindex namespaces
-When looking up names, the reader also considers ``shorthands''. If
-the programmer supplied them, this allows the reader to find a symbol
-even if its name isn't typed out fully in the source code. Of course,
-the reader needs to be aware of some pre-established context about
-such shorthands, much as one needs context to be to able to refer
-uniquely to Jan Jones by just the name ``Jan'': it's probably fine
-when amongst the Joneses, or when Jan has been mentioned recently, but
-very ambiguous in any other situation. @xref{Shorthands}.
+ When the Lisp reader encounters a name that references a symbol in
+the source code, it reads all the characters of that name. Then it
+looks up that name in a table called an @dfn{obarray} to find the
+symbol that the programmer meant. The technique used in this lookup
+is called ``hashing'', an efficient method of looking something up by
+converting a sequence of characters to a number, known as a ``hash
+code''. 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 @dfn{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 @ref{Hash Tables}.)
+
+When looking up names, the Lisp reader also considers ``shorthands''.
+If the programmer supplied them, this allows the reader to find a
+symbol even if its name isn't present in its full form in the source
+code. Of course, the reader needs to be aware of some pre-established
+context about such shorthands, much as one needs context to be to able
+to refer uniquely to Jan Jones by just the name ``Jan'': it's probably
+fine when amongst the Joneses, or when Jan has been mentioned
+recently, but very ambiguous in any other situation.
+@xref{Shorthands}.
@cindex interning
If a symbol with the desired name is found, the reader uses that
names, as long as you keep reading with the same obarray.
Interning usually happens automatically in the reader, but sometimes
-other programs may want to do it. For example, a hypothetical
-telephone book program could intern the name of each looked up
-person's name as a symbol, even if the obarray did not contain it, so
-that it could attach information to that new symbol such as the last
-time someone looked it up.
+other programs may want to do it. For example, after the @kbd{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.
+As another example, a hypothetical telephone book program could intern
+the name of each looked up person's name as a symbol, even if the
+obarray did not contain it, so that it could attach information to
+that new symbol, such as the last time someone looked it up.
@cindex symbol equality
@cindex uninterned symbol
obarray. They are called @dfn{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.
+value of a variable. Uninterned symbols are sometimes useful in
+generating Lisp code, see below.
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
@b{Common Lisp note:} Unlike Common Lisp, Emacs Lisp does not provide
for interning the same name in several different ``packages'', thus
creating multiple symbols with the same name but different packages.
-Emacs Lisp provides a different namespacing system called shorthands
-()
+Emacs Lisp provides a different namespacing system called
+``shorthands'' (@pxref{Shorthands}).
@end quotation
Most of the functions below take a name and sometimes an obarray as
do it!
@end defun
-@cindex uninterned symbol (generated code)
+@cindex uninterned symbol, and generating Lisp code
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.
@defun gensym &optional prefix
This function returns a symbol using @code{make-symbol}, whose name is
-made by appending @code{gensym-counter} to @var{prefix} and increnting
-that counter, guaranteeing that no two calls to this function
-generates a symbol with the same name. The prefix defaults to
+made by appending @code{gensym-counter} to @var{prefix} and incrementing
+that counter, guaranteeing that no two calls to this function will
+generate a symbol with the same name. The prefix defaults to
@code{"g"}.
@end defun
-@cindex uninterned symbol (recommendation for generated code)
To avoid problems when accidentally interning printed representation
-of generated code, (@pxref{Printed Representation}), it is recommended
+of generated code (@pxref{Printed Representation}), it is recommended
to use @code{gensym} instead of @code{make-symbol}.
@defun intern name &optional obarray
@section Shorthands
@cindex shorthands
@cindex symbolic shorthands
+@cindex namespacing
+@cindex namespaces
-@dfn{Shorthands}, sometimes known as "renamed symbols", are symbolic
-forms found in Lisp source. They're just like regular symbolic forms,
-except that when the Lisp reader encounters them, it produces symbols
-which have a different and usually longer @dfn{print name}
-(@pxref{Symbol Components}).
+ The symbol @dfn{shorthands}, sometimes known as ``renamed symbols'', are
+symbolic forms found in Lisp source. They're just like regular
+symbolic forms, except that when the Lisp reader encounters them, it
+produces symbols which have a different and usually longer @dfn{print
+name} (@pxref{Symbol Components}).
It is useful to think of shorthands as @emph{abbreviating} the full
-names of intended symbols. Despite this, do not confuse with the
+names of intended symbols. Despite this, do not confuse shorthands with the
Abbrev system @pxref{Abbrevs}.
@cindex namespace etiquette
-Shorthands make Emacs Lisp's namespacing etiquette easier to work
+Shorthands make Emacs Lisp's @dfn{namespacing etiquette} easier to work
with. Since all symbols are stored in a single obarray
(@pxref{Creating Symbols}), programmers commonly prefix each symbol
name with the name of the library where it originates. For example,
the functions @code{text-property-search-forward} and
@code{text-property-search-backward} both belong to the
-@code{text-property-search.el} library (@pxref{Loading}). By properly
+@file{text-property-search.el} library (@pxref{Loading}). By properly
prefixing symbol names, one effectively prevents clashes between
-similarly named symbols which belong to different libraries and do
+similarly named symbols which belong to different libraries and thus do
different things. However, this practice commonly originates very
-long symbols names, which are bothersome to type and read after a
-while.
+long symbols names, which are inconvenient to type and read after a
+while. Shorthands solve these issues in a clean way.
@defvar elisp-shorthands
-This variable is an alist whose elements have the form
+This variable's value is an alist whose elements have the form
@code{(@var{shorthand-prefix} . @var{longhand-prefix})}. Each element
instructs the Lisp reader to read every symbol form which starts with
@var{shorthand-prefix} as if it started with @var{longhand-prefix}
instead.
-This variable may only be set file-locally (@pxref{File Variables, ,
+This variable may only be set in file-local variables (@pxref{File Variables, ,
Local Variables in Files, emacs, The GNU Emacs Manual}).
@end defvar
-Take this excerpt from following example of a hypothetical string
+Here's an example of shorthands usage in a hypothetical string
manipulating library @file{some-nice-string-utils.el}.
-@example
+@smalllisp
(defun some-nice-string-utils-split (separator s &optional omit-nulls)
- "A match-data saving variation on `split-string'."
+ "A match-data saving variant of `split-string'."
(save-match-data (split-string s separator omit-nulls)))
(defun some-nice-string-utils-lines (s)
- "Split string S into a list of strings on newline characters."
+ "Split string S at newline characters into a list of strings."
(some-nice-string-utils-split "\\(\r\n\\|[\n\r]\\)" s))
-@end example
+@end smalllisp
As can be seen, it's quite tedious to read or develop this code since
-the symbol names to type are so long. We can use shorthands to good
-effect here.
+the symbol names to type are so long. We can use shorthands to
+alleviate that.
-@example
+@lisp1
(defun snu-split (separator s &optional omit-nulls)
"A match-data saving variation on `split-string'."
(save-match-data (split-string s separator omit-nulls)))
;; Local Variables:
;; elisp-shorthands: (("snu-" . "some-nice-string-utils-"))
;; End:
-@end example
+@end lisp
Even though the two excerpts look different, they are quite identical
-after Lisp reader is done with them. Both will lead to the very same
+after the Lisp reader processes them. Both will lead to the very same
symbols being interned (@pxref{Creating Symbols}). Thus loading or
byte-compiling any of the two files has equivalent results. The
shorthands @code{snu-split} and @code{snu-lines} used in the second
that multiple libraries depending on
@file{some-nice-string-utils-lines.el} refer to the same symbols under
@emph{different} shorthands, or not using shorthands at all. In the
-next example, the @file{my-tricks.el} library refers to symbol
-aforementioned symbol @code{some-nice-string-utils-lines} using the
-@code{sns-} prefix.
+next example, the @file{my-tricks.el} library refers to the
+symbol @code{some-nice-string-utils-lines} using the
+@code{sns-} prefix instead of @code{snu-}.
@example
(defun t-reverse-lines (s) (string-join (reverse (sns-lines s)) "\n")
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