clojure.core/cond->

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Type: macro Added: Clojure 1.5 Examples: 7 Runnable: 4

([expr & clauses])

Takes an expression and a set of test/form pairs. Threads expr (via ->) through each form for which the corresponding test expression is true. Note that, unlike cond branching, cond-> threading does not short circuit after the first true test expression.

Examples

by avasenin on 2014-10-13 ▶

(cond-> 1 ; we start with 1 true inc ; the condition is true so (inc 1) => 2 false (* 42) ; the condition is false so the operation is skipped (= 2 2) (* 3)) ; (= 2 2) is true so (* 2 3) => 6 ;;=> 6 ;; notice that the threaded value gets used in ;; only the form and not the test part of the clause.

by arichiardi on 2014-10-26 . May have evaluation errors. ▶

;; Useful when you want to conditionally evaluate expressions and thread them ;; together. For instance, the following returns a vector containing the names ;; (as symbols) of the implementing classes of obj. => (defn instance->types [obj] (cond-> [] (instance? java.util.SortedMap obj) (conj 'SortedMap) (instance? java.util.AbstractMap obj) (conj 'AbstractMap))) #'user/instance->types => (def hm (java.util.HashMap.)) #'user/hm => (instance->types hm) [AbstractMap] => (def tm (java.util.TreeMap.)) #'user/tm => (instance->types tm) [SortedMap AbstractMap]

by trikitrok on 2016-04-11 . May have evaluation errors. ▶

=> (defn divisible-by? [divisor number] (zero? (mod number divisor))) #'user/divisible-by? => (defn say [n] (cond-> nil (divisible-by? 3 n) (str "Fizz") (divisible-by? 5 n) (str "Buzz") :always (or (str n)))) #'user/say => (say 1) "1" => (say 3) "Fizz" => (say 5) "Buzz" => (say 15) "FizzBuzz"

by mhmdsalem1993 on 2016-12-27 ▶

(let [x 1 y 2] (cond-> [] (odd? x) (conj "x is odd") (zero? (rem y 3)) (conj "y is divisible by 3") (even? y) (conj "y is even"))) ;=> ["x is odd" "y is even"] ;;; IS Equivalent to (let [x 1 y 2] (as-> [] <> (if (odd? x) (conj <> "x is odd") <>) (if (zero? (rem y 3)) (conj <> "y is divisible by 3") <>) (if (even? y) (conj <> "y is even") <>))) ;=> ["x is odd" "y is even"]

by reborg on 2018-11-12 . May have evaluation errors. ▶

; Consider a code snippet that coerces any string to an integer, else noop: (let [x "123"] (if (string? x) (Integer. x) x)) ; We can reduce the repetition of `x` by using `cond->` (let [x "123"] (cond-> x (string? x) (Integer.)))

by rainbyte on 2022-07-27 ▶

;; always returns 1 (defn f [x] 1) ;; cond-> doesn't short circuit on nil (cond-> nil true f false inc true inc) ;=> 2

by daveliepmann on 2023-04-26 ▶

;; If most of a -> is unconditional but one step is conditional, cond-> is useful: (-> {} (assoc :foo "foo") (cond-> false (assoc :truthy true)) (assoc :bar "bar")) ;; => {:foo "foo", :bar "bar"} (-> {} (assoc :foo "foo") (cond-> true ; this is the only change from above (assoc :truthy true)) (assoc :bar "bar")) ;; => {:foo "foo", :truthy true, :bar "bar"}

See also

• clojure.core/cond->>
• clojure.core/-> (4)
• clojure.core/->> (7)
• clojure.core/as-> (4)
• clojure.core/some-> (0)
• clojure.core/some->>
• clojure.core/cond (5)
• clojure.core/if (2)

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