@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990-1995, 1998-1999, 2001-2013 Free Software
+@c Copyright (C) 1990-1995, 1998-1999, 2001-2014 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Control Structures
@code{cond} tries the clauses in textual order, by evaluating the
@var{condition} of each clause. If the value of @var{condition} is
non-@code{nil}, the clause ``succeeds''; then @code{cond} evaluates its
-@var{body-forms}, and the value of the last of @var{body-forms} becomes
-the value of the @code{cond}. The remaining clauses are ignored.
+@var{body-forms}, and returns the value of the last of @var{body-forms}.
+Any remaining clauses are ignored.
If the value of @var{condition} is @code{nil}, the clause ``fails'', so
-the @code{cond} moves on to the following clause, trying its
-@var{condition}.
-
-If every @var{condition} evaluates to @code{nil}, so that every clause
-fails, @code{cond} returns @code{nil}.
+the @code{cond} moves on to the following clause, trying its @var{condition}.
A clause may also look like this:
@end example
@noindent
-Then, if @var{condition} is non-@code{nil} when tested, the value of
-@var{condition} becomes the value of the @code{cond} form.
+Then, if @var{condition} is non-@code{nil} when tested, the @code{cond}
+form returns the value of @var{condition}.
+
+If every @var{condition} evaluates to @code{nil}, so that every clause
+fails, @code{cond} returns @code{nil}.
The following example has four clauses, which test for the cases where
the value of @code{x} is a number, string, buffer and symbol,
was returned by @code{(get-return-code x)}.
To give a more complex example, a simple interpreter for a little
-expression language could look like:
+expression language could look like (note that this example requires
+lexical binding):
@example
(defun evaluate (exp env)
(pcase exp
(`(add ,x ,y) (+ (evaluate x env) (evaluate y env)))
- (`(call ,fun ,arg) (funcall (evaluate fun) (evaluate arg env)))
+ (`(call ,fun ,arg) (funcall (evaluate fun env) (evaluate arg env)))
(`(fn ,arg ,body) (lambda (val)
(evaluate body (cons (cons arg val) env))))
((pred numberp) exp)
@code{(pred numberp)} is a pattern that simply checks that @code{exp}
is a number, and @code{_} is the catch-all pattern that matches anything.
+Here are some sample programs including their evaluation results:
+
+@example
+(evaluate '(add 1 2) nil) ;=> 3
+(evaluate '(add x y) '((x . 1) (y . 2))) ;=> 3
+(evaluate '(call (fn x (add 1 x)) 2) nil) ;=> 3
+(evaluate '(sub 1 2) nil) ;=> error
+@end example
+
There are two kinds of patterns involved in @code{pcase}, called
@emph{U-patterns} and @emph{Q-patterns}. The @var{upattern} mentioned above
are U-patterns and can take the following forms: