21.2 Lists and Sexps

By convention, Emacs keys for dealing with balanced expressions are usually Control-Meta- characters. They tend to be analogous in function to their Control- and Meta- equivalents. These commands are usually thought of as pertaining to expressions in programming languages, but can be useful with any language in which some sort of parentheses exist (including English).

The commands fall into two classes. Some commands deal only with lists (parenthetical groupings). They see nothing except parentheses, brackets, braces (depending on what must balance in the language you are working with), and escape characters that might be used to quote those.

The other commands deal with expressions or sexps. The word ‘sexp’ is derived from s-expression, the term for a symbolic expression in Lisp. In Emacs, the notion of ‘sexp’ is not limited to Lisp. It refers to an expression in the language your program is written in. Each programming language has its own major mode, which customizes the syntax tables so that expressions in that language count as sexps.

Sexps typically include symbols, numbers, and string constants, as well as anything contained in parentheses, brackets, or braces.

In languages that use prefix and infix operators, such as C, it is not possible for all expressions to be sexps. For example, C mode does not recognize ‘foo + bar’ as an sexp, even though it is a C expression; it recognizes ‘foo’ as one sexp and ‘bar’ as another, with the ‘+’ as punctuation between them. This is a fundamental ambiguity: both ‘foo + bar’ and ‘foo’ are legitimate choices for the sexp to move over if point is at the ‘f’. Note that ‘(foo + bar)’ is a sexp in C mode.

Some languages have obscure forms of syntax for expressions that nobody has bothered to make Emacs understand properly.

C-M-f

Move forward over an sexp (forward-sexp).

C-M-b

Move backward over an sexp (backward-sexp).

C-M-k

Kill sexp forward (kill-sexp).

C-M-u

Move up and backward in list structure (backward-up-list).

C-M-d

Move down and forward in list structure (down-list).

C-M-n

Move forward over a list (forward-list).

C-M-p

Move backward over a list (backward-list).

C-M-t

Transpose expressions (transpose-sexps).

C-M-@

Put mark after following expression (mark-sexp).

To move forward over an sexp, use C-M-f (forward-sexp). If the first significant character after point is an opening delimiter (‘(’ in Lisp; ‘(’, ‘[’, or ‘{’ in C), C-M-f moves past the matching closing delimiter. If the character begins a symbol, string, or number, C-M-f moves over that. If the character after point is a closing delimiter, C-M-f just moves past it. (This last is not really moving across an sexp; it is an exception which is included in the definition of C-M-f because it is as useful a behavior as anyone can think of for that situation.)

The command C-M-b (backward-sexp) moves backward over a sexp. The detailed rules are like those above for C-M-f, but with directions reversed. If there are any prefix characters (single quote, back quote, and comma, in Lisp) preceding the sexp, C-M-b moves back over them as well.

C-M-f or C-M-b with an argument repeats that operation the specified number of times; with a negative argument, it moves in the opposite direction.

Killing an sexp at a time can be done with C-M-k (kill-sexp). C-M-k kills the characters that C-M-f would move over.

The list commands, C-M-n (forward-list) and C-M-p (backward-list), move over lists like the sexp commands but skip over any number of other kinds of sexps (symbols, strings, etc). In some situations, these commands are useful because they usually ignore comments, since the comments usually do not contain any lists.

C-M-n and C-M-p stay at the same level in parentheses, when that is possible. To move up one (or n) levels, use C-M-u (backward-up-list). C-M-u moves backward up past one unmatched opening delimiter. A positive argument serves as a repeat count; a negative argument reverses direction of motion and also requests repetition, so it moves forward and up one or more levels.

To move down in list structure, use C-M-d (down-list). In Lisp mode, where ‘(’ is the only opening delimiter, this is nearly the same as searching for a ‘(’. An argument specifies the number of levels of parentheses to go down.

C-M-t (transpose-sexps) drags the previous sexp across the next one. An argument serves as a repeat count, and a negative argument drags backwards (thus canceling out the effect of C-M-t with a positive argument). An argument of zero, rather than doing nothing, transposes the sexps ending after point and the mark.

To make the region be the next sexp in the buffer, use C-M-@ (mark-sexp) which sets the mark at the same place that C-M-f would move to. C-M-@ takes arguments like C-M-f. In particular, a negative argument is useful for putting the mark at the beginning of the previous sexp.

The list and sexp commands’ understanding of syntax is completely controlled by the syntax table. Any character can, for example, be declared to be an opening delimiter and act like an open parenthesis. See Syntax.