Guards in Patterns

Guards provide additional constraints on patterns:

;; Simple guard
(case x
  (n (when (> n 10)) 'big)
  (n (when (> n 0)) 'small)
  (_ 'other))

;; Multiple conditions
(case x
  (n (when (> n 0) (< n 100))
   'in-range))

;; Guard functions
(case x
  (x (when (is_atom x)) 'atom)
  (x (when (is_list x)) 'list)
  (_ 'other))

;; Complex guards
(case (tuple x y)
  ((tuple a b) (when (> a 0) (> b 0) (== (+ a b) 10))
   'valid))

Allowed in Guards:

• Type tests: is_atom/1, is_list/1, etc.
• Comparisons: ==, <, >, etc.
• Arithmetic: +, -, *, div, rem, etc.
• Boolean: and, or, not, andalso, orelse
• BIFs: abs/1, element/2, tuple_size/1, etc.

Not Allowed in Guards:

• User-defined functions
• Send/receive operations
• Most BIFs (only guard-safe ones allowed)