Next: List Commands Prev: Program Modes Up: Programs
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 human languages).
These commands fall into two classes. Some deal only with "lists"
(parenthetical groupings). They see nothing except parentheses,
brackets, braces (whichever ones 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 ancient term for an expression in
Lisp. But in Emacs, the notion of `sexp' is not limited to Lisp. It
refers to an expression in whatever 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 a 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 single sexp in C
Some languages have obscure forms of syntax for expressions that
nobody has bothered to make Emacs understand properly.
automatically generated by info2www