A predicate is a function used to decide whether a value passes a condition. In Clojure, its result need not be a Boolean: nil and false count as false, while other results count as true. APIs such as filter, every?, and some call a predicate with one collection item at a time. section 2.2.9 section 8.1.5
Choose the operation according to the question:
- filter keeps items that pass; remove discards them. Both return lazy sequences. Prefer predicates without side effects: lazy processing does not guarantee a once-only call for each item. section 3.5.3
- every? asks whether all items pass; not-every? asks whether at least one fails. some looks for a passing result; not-any? asks whether none pass. The question-mark functions return Booleans, while some can return the predicate’s result. section 8.1.5
- every-pred combines checks with AND; some-fn combines them with OR. Each produces a function that can test zero or more values, stopping when the answer is determined. every-pred returns a Boolean; some-fn can return a non-Boolean match. section 2.2.9
Select and combine checks
(filter odd? [1 2 3 4 5]) ;; (1 3 5) (defn symmetric? [xs] (= (seq xs) (reverse xs))) (def palindrome? (every-pred some? string? not-empty symmetric?)) (filter palindrome? ["a" nil :abba 1 "" "racecar"]) ;; ("a" "racecar") The combined predicate excludes nil, non-strings, and empty strings before checking symmetry. Passing (and number? pos? odd?) directly to remove would not combine those functions per item: and evaluates the function values when that expression is formed. Use every-pred, or an anonymous function that calls each check on its item. section 3.5.3 section 2.2.9
A useful predicate can return evidence rather than just true. A set used as a function can return a matching member, and a map used as a function can return an associated value. For example, some can return a prize description found while checking tickets; some-fn can return the unwanted word found while checking email. These results can be used immediately rather than repeating the search. section 8.1.5 section 2.2.9
Predicates also express function contracts. In defn, :pre predicates run before the body and can inspect arguments; :post predicates run afterward and can inspect the return value through %. Failed conditions raise an AssertionError. Keep the distinction clear: a predicate supplies a condition, but the function using it determines whether the result becomes a Boolean, a selected item, or a returned match. section 2.1.1 section 2.2.9
