Special Patterns

This section contains patterns that has specific roles in itself.

Place Pattern

Syntax

place variable

Just the same as using variable as a variable pattern, except the variable is accessed by symbol-macrolet instead of let. This allows the use of setf to modify the matched object.

Example:

(defvar *x* (list 0 1))
(match *x*
  ((list (place x) y)
   (setf x :success)
   (setf y :fail)))

(print *x*)
;; --> (:SUCCESS 1)

Bind Pattern

Syntax

<> pattern value &optional var

The current matching value is bound to var. The result of evaluating value using var is then matched against pattern. var is optional and can be omitted when value is a constant and does not need the current matching value.

This is important when you write a defpattern that has a default value. Consider writing a pattern that matches against both of 'string and '(string *) and has a subpattern length. length should be bound to '* even when the input is 'string. With <> pattern, it can be implemented as below.

(defpattern string-type-specifier (length)
   `(or (list 'string ,length)
        (and 'string (<> ,length '*))))

Access Pattern

Just want to access an element? It’s time to use access pattern:

Syntax

access #’accessor subpattern

Syntax

access ‘accessor subpattern

accessor

a function name.

The object is not checked. The value of funcall ing the current object is matched against subpattern.

Example:

(match '((1 2 (3 4)) 5 (6))
  ((access #'flatten (list* _ _ 3 _))))