Commit f433306a authored by Stefan Monnier's avatar Stefan Monnier
Browse files

* doc/lispref/control.texi (Pattern maching case statement): New node.

parent 2c066ad3
2012-12-09 Stefan Monnier <monnier@iro.umontreal.ca>
* control.texi (Pattern maching case statement): New node.
2012-12-06 Stefan Monnier <monnier@iro.umontreal.ca>
* customize.texi (Variable Definitions): Mention the default :group
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......@@ -285,6 +285,110 @@ For example:
@end group
@end example
@menu
* Pattern maching case statement::
@end menu
@node Pattern maching case statement
@subsection Pattern maching case statement
@cindex pcase
@cindex pattern matching
To compare a particular value against various possible cases, the macro
@code{pcase} can come handy. It takes the following form:
@example
(pcase @var{exp} @var{branch}1 @var{branch}2 @var{branch}3 @dots{})
@end example
where each @var{branch} takes the form @code{(@var{upattern}
@var{body-forms}@dots{})}.
It will first evaluate @var{exp} and then compare the value against each
@var{upattern} to see which @var{branch} to use, after which it will run the
corresponding @var{body-forms}. A common use case is to distinguish
between a few different constant values:
@example
(pcase (get-return-code x)
(`success (message "Done!"))
(`would-block (message "Sorry, can't do it now"))
(`read-only (message "The shmliblick is read-only"))
(`access-denied (message "You do not have the needed rights"))
(code (message "Unknown return code %S" code)))
@end example
In the last clause, @code{code} is a variable that gets bound to the value that
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:
@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)))
(`(fn ,arg ,body) (lambda (val)
(evaluate body (cons (cons arg val) env))))
((pred numberp) exp)
((pred symbolp) (cdr (assq exp env)))
(_ (error "Unknown expression %S" exp))))
@end example
Where @code{`(add ,x ,y)} is a pattern that checks that @code{exp} is a three
element list starting with the symbol @code{add}, then extracts the second and
third elements and binds them to the variables @code{x} and @code{y}.
@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.
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:
@table @code
@item `@var{qpattern}
This is one of the most common form of patterns. The intention is to mimic the
backquote macro: this pattern matches those values that could have been built
by such a backquote expression. Since we're pattern matching rather than
building a value, the unquote does not indicate where to plug an expression,
but instead it lets one specify a U-pattern that should match the value at
that location.
More specifically, a Q-pattern can take the following forms:
@table @code
@item (@var{qpattern1} . @var{qpattern2})
This pattern matches any cons cell whose @code{car} matches @var{QPATTERN1} and
whose @code{cdr} matches @var{PATTERN2}.
@item @var{atom}
This pattern matches any atom @code{equal} to @var{atom}.
@item ,@var{upattern}
This pattern matches any object that matches the @var{upattern}.
@end table
@item @var{symbol}
A mere symbol in a U-pattern matches anything, and additionally let-binds this
symbol to the value that it matched, so that you can later refer to it, either
in the @var{body-forms} or also later in the pattern.
@item _
This so-called @emph{don't care} pattern matches anything, like the previous
one, but unless symbol patterns it does not bind any variable.
@item (pred @var{pred})
This pattern matches if the function @var{pred} returns non-@code{nil} when
called with the object being matched.
@item (or @var{upattern1} @var{upattern2}@dots{})
This pattern matches as soon as one of the argument patterns succeeds.
All argument patterns should let-bind the same variables.
@item (and @var{upattern1} @var{upattern2}@dots{})
This pattern matches only if all the argument patterns succeed.
@item (guard @var{exp})
This pattern ignores the object being examined and simply succeeds if @var{exp}
evaluates to non-@code{nil} and fails otherwise. It is typically used inside
an @code{and} pattern. For example, @code{(and x (guard (< x 10)))}
is a pattern which matches any number smaller than 10 and let-binds it to
the variable @code{x}.
@end table
@node Combining Conditions
@section Constructs for Combining Conditions
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