Logic Based Patterns

And, Or pattern

Syntax

and &rest subpattterns

or &rest subpattterns

They match when all/some of the subpatterns matches against the element. For example,

(match x
  ((or (list 1 a)
       (cons a 3))
   a))

matches against both (1 2) and (4 . 3) and returns 2 and 4, respectively. Also,

(match x
  ((and (list 1 _)
        (list _ 2))
   t))

is same as below.

(match x
  ((list 1 2)
   t))

Not pattern

Syntax

not subpattern

It does not match when subpattern matches. The variables used in subpattern is not visible in the body.

Guard pattern

Syntax

guard subpattern1 test-form {generator-form subpattern2}*

test-form

a predicate form, evaluated.

generator-form

a form that produce a value, which are then matched against the next subpattern2.

The object is first matched against subpattern1. If that fails, the whole match fails. Otherwise, test-form is evaluated. When the result is true, then each of generator-form is evaluated and matched against corresponding subpattern2.

If subpattern1 is of the form (or <pat1> <pat2> ...) however, it tries to match with one of the or-subpatterns such that test-form is satisfied.

Example:

(match (list 2 5)
  ((guard (list x y)     ; subpattern1
          (= 10 (* x y))) ; test-form
   t))
;; --> t

(match '(1)
  ((guard (or (list x) x)     ; subpattern1
          (evenp x)) ; test-form
   x))
;; --> error, since '(1) gets matched with x, after failing evenp on matching (list x)