An important function of each major mode is to customize the TAB key to indent properly for the language being edited. This section describes the mechanism of the TAB key and how to control it. The functions in this section return unpredictable values.
This is the command bound to TAB in most editing modes. Its usual action is to indent the current line, but it can alternatively insert a tab character or indent a region.
Here is what it does:
indent-regionto indent all the text in the region (see Indenting an Entire Region).
indent-to-left-margin(a trivial command that inserts a tab character), or if the variable
tab-always-indentspecifies that a tab character ought to be inserted (see below), then it inserts a tab character.
indent-line-function. If the line is already indented, and the value of
complete(see below), it tries completing the text at point.
If rigid is non-
nil (interactively, with a prefix
argument), then after this command indents a line or inserts a tab, it
also rigidly indents the entire balanced expression which starts at
the beginning of the current line, in order to reflect the new
indentation. This argument is ignored if the command indents the
This variable’s value is the function to be used by
indent-for-tab-command, and various other indentation commands,
to indent the current line. It is usually assigned by the major mode;
for instance, Lisp mode sets it to
lisp-indent-line, C mode
sets it to
c-indent-line, and so on. The default value is
indent-relative. See Automatic Indentation of code.
This command calls the function in
indent the current line in a way appropriate for the current major mode.
This function inserts a newline, then indents the new line (the one
following the newline just inserted) according to the major mode. It
does indentation by calling
This command reindents the current line, inserts a newline at point,
and then indents the new line (the one following the newline just
inserted). It does indentation on both lines by calling
This variable can be used to customize the behavior of the TAB
indent-for-tab-command) command. If the value is
(the default), the command normally just indents the current line. If
the value is
nil, the command indents the current line only if
point is at the left margin or in the line’s indentation; otherwise,
it inserts a tab character. If the value is
command first tries to indent the current line, and if the line was
already indented, it calls
completion-at-point to complete the
text at point (see Completion in Ordinary Buffers).
complete, whether to expand or
indent can be further customized via the
variable. The following values can be used:
Only complete if point is at the end of a line.
Complete unless the next character has word syntax.
Complete unless the next character has word syntax or is a parenthesis.
Complete unless the next character has word syntax, or is a parenthesis, or is punctuation.
In any case, typing TAB a second time always results in completion.
Some major modes need to support embedded regions of text whose
syntax belongs to a different major mode. Examples include
literate programming source files that combine documentation and
snippets of source code, Yacc/Bison programs that include snippets of
Python or JS code, etc. To correctly indent the embedded chunks, the primary
mode needs to delegate the indentation to another mode’s indentation
engine (e.g., call
js-indent-line for JS code or
python-indent-line for Python), while providing it with some
context to guide the indentation. Major modes, for their part, should
widen in their indentation code and obey
This variable, when non-
nil, holds the indentation context for
the sub-mode’s indentation engine provided by the superior major mode.
The value should be a list of the form
(first-column . rest.
The members of the list have the following meaning:
The column to be used for top-level constructs. This replaces the default value of the top-level column used by the sub-mode, usually zero.
This value is currently unused.
The following convenience function should be used by major mode’s indentation engine in support of invocations as sub-modes of another major mode.
Call this function instead of using a literal value (usually, zero) of the column number for indenting top-level program constructs. The function’s value is the column number to use for top-level constructs. When no superior mode is in effect, this function returns zero.