21.4.3 Customizing Lisp Indentation
The indentation pattern for a Lisp expression can depend on the function
called by the expression. For each Lisp function, you can choose among
several predefined patterns of indentation, or define an arbitrary one
with a Lisp program.
The standard pattern of indentation is as follows: the second line
of the expression is indented under the first argument, if that is on
the same line as the beginning of the expression; otherwise, the second
line is indented underneath the function name. Each following line is
indented under the previous line whose nesting depth is the same.
If the variable `lisp-indent-offset' is non-`nil', it overrides the
usual indentation pattern for the second line of an expression, so that
such lines are always indented `lisp-indent-offset' more columns than
the containing list.
Certain functions override the standard pattern. Functions whose
names start with `def' always indent the second line by
`lisp-body-indention' extra columns beyond the open-parenthesis
starting the expression.
Individual functions can override the standard pattern in various
ways, according to the `lisp-indent-function' property of the function
name. (Note: `lisp-indent-function' was formerly called
`lisp-indent-hook'). There are four possibilities for this property:
This is the same as no property; the standard indentation pattern
The pattern used for function names that start with `def' is used
for this function also.
a number, NUMBER
The first NUMBER arguments of the function are "distinguished"
arguments; the rest are considered the "body" of the expression.
A line in the expression is indented according to whether the
first argument on it is distinguished or not. If the argument is
part of the body, the line is indented `lisp-body-indent' more
columns than the open-parenthesis starting the containing
expression. If the argument is distinguished and is either the
first or second argument, it is indented twice that many extra
columns. If the argument is distinguished and not the first or
second argument, the standard pattern is followed for that line.
a symbol, SYMBOL
SYMBOL should be a function name; that function is called to
calculate the indentation of a line within this expression. The
function receives two arguments:
The value returned by `parse-partial-sexp' (a Lisp primitive
for indentation and nesting computation) when it parses up to
the beginning of this line.
The position at which the line being indented begins.
It should return either a number, which is the number of columns of
indentation for that line, or a list whose first element is such a
number. The difference between returning a number and returning a
list is that a number says that all following lines at the same
nesting level should be indented just like this one; a list says
that following lines might call for different indentations. This
makes a difference when the indentation is computed by `C-M-q'; if
the value is a number, `C-M-q' need not recalculate indentation
for the following lines until the end of the list.
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