clojure.core/defn

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Type: macro Added: Clojure 1.0 Examples: 11 Runnable: 5

([name doc-string? attr-map? [params*] prepost-map? body] [name doc-string? attr-map? ([params*] prepost-map? body) + attr-map?])

Same as (def name (fn [params* ] exprs*)) or (def name (fn ([params* ] exprs*)+)) with any doc-string or attrs added to the var metadata. prepost-map defines a map with optional keys :pre and :post that contain collections of pre or post conditions.

Examples

by nipra on 2010-07-15 . May have evaluation errors. ▶

(defn foo [a b c] (* a b c)) #'user/foo (foo 1 2 3) 6 (defn bar [a b & [c]] (if c (* a b c) (* a b 100))) #'user/bar (bar 5 6) 3000 (bar 5 6 2) 60 (defn baz [a b & {:keys [c d] :or {c 10 d 20}}] (* a b c d)) #'user/baz (baz 2 3) 1200 (baz 2 3 :c 5) 600 (baz 2 3 :c 5 :d 6) 180 (defn boo [a b & {:keys [c d] :or {c 10 d 20} :as all-specified}] (println all-specified) (* a b c d)) #'user/boo (boo 2 3) nil 1200 (boo 2 3 :c 5) {:c 5} 600 (boo 1 2 :d 3 :c 4) {:c 4, :d 3} 24

by kotarak on 2010-07-15 . May have evaluation errors. ▶

(defn bar ([a b] (bar a b 100)) ([a b c] (* a b c))) #'user/bar (bar 5 6) 3000 (bar 5 6 2) 60

by pkolloch on 2010-07-30 ▶

;; You can use destructuring to have keyword arguments. This would be a ;; pretty verbose version of map (in an example a bit more verbose than ;; the first above): (defn keyworded-map [& {function :function sequence :sequence}] (map function sequence)) ;; You can call it like this: (keyworded-map :sequence [1 2 3] :function #(+ % 2)) (3 4 5) ;; The declaration can be shortened with ":keys" if your local variables ;; should be named in the same way as your keys in the map: (defn keyworded-map [& {:keys [function sequence]}] (map function sequence))

by AtKaaZ on 2013-02-19 . May have evaluation errors. ▶

(defn somefn [req1 req2 ;required params & {:keys [a b c d e] ;optional params :or {a 1 ;optional params with preset default values other than the nil default ; b takes nil if not specified on call c 3 ; c is 3 when not specified on call d 0 ; d is 0 --//-- ; e takes nil if not specified on call } :as mapOfParamsSpecifiedOnCall ;takes nil if no extra params(other than the required ones) are specified on call }] (println req1 req2 mapOfParamsSpecifiedOnCall a b c d e) ) => (somefn 9 10 :b 2 :d 4) ;9 10 {:b 2, :d 4} 1 2 3 4 nil nil => (somefn) ;ArityException Wrong number of args (0) passed to: funxions$somefn ;clojure.lang.AFn.throwArity (AFn.java:437) => (somefn 9 10) ;9 10 nil 1 nil 3 0 nil nil => (somefn 9 10 :x 123) ;9 10 {:x 123} 1 nil 3 0 nil nil => (somefn 9 10 123) ;IllegalArgumentException No value supplied for key: 123 ;clojure.lang.PersistentHashMap.create (PersistentHashMap.java:77) => (somefn 9 10 123 45) ;9 10 {123 45} 1 nil 3 0 nil nil => (try (somefn 9 10 123) (catch IllegalArgumentException e (println "caught:" e))) ;caught: #<IllegalArgumentException java.lang.IllegalArgumentException: No value supplied for key: 123> nil

by dxlr8r on 2015-09-18 ▶

;; :as only include parameters provided, not the default (:or) ones. ;; This is some boilerplate code to get around this. ;; Hopefully not needed in the future revisions of Clojure. (defn bar [f g h & {:keys [override]}] (let [default {:a 1 :b 2 :c 3} args (merge default override)] (conj '() f g h args))) (bar 1 2 3 :override {:a 9 :z 5}) ; returns -> ({:z 5, :a 9, :b 2, :c 3} 3 2 1)

by ibercode on 2018-01-18 ▶

;;defn basic examples (defn say-hi [name] (str "Hi " name)) (say-hi "Jack") ;;"Hi Jack" ;;the same result using def (def say-hello (fn [name] (str "Hello " name))) (say-hello "Bob") ;;"Hello Bob" ;;the same result using def and an anonymous function (def say-bye #(str "Bye Bye " %)) (say-bye "Mark") ;;"Bye Bye Mark"

by jakubholynet on 2018-01-23 . May have evaluation errors. ▶

;;define a function with metadata (defn hello {:awesome true} [] nil) (meta #'hello) => {:arglists ([]), :awesome true, ... } ;; define a function with a return value type hint (defn hinted ^long [] 42) (-> #'hinted meta :arglists first meta :tag) => long ;; both metadata on the fn and return type hint (on the argument vector) (defn hinted+meta {:awesome true} ^long [] 42)

by victorb on 2019-01-13 . May have evaluation errors. ▶

;; Documentation can also be added with `defn` => (defn add "Adds two numbers" [x y] (+ x y)) #'user/add ;; This documentation can be read with the `doc` function => (doc add) ------------------------- user/add ([x y]) Adds two numbers ;; It can also be used when searching for functions with `find-doc` => (find-doc "two numbers") ------------------------- user/add ([x y]) Adds two numbers

by cljlc on 2019-02-20 ▶

;; prepost-map examples ;; check that exactly one of :a or :b are in the argument map: (defn example [{:keys [a b]}] {:pre [(not (and a b)) (or a b)]} (println a b)) ;; #'user/example (example {:a 1 :b 2}) ;; Execution error (AssertionError) at user/example (REPL:1). ;; Assert failed: (not (and a b)) (example {:c 1 :d 2}) ;; Execution error (AssertionError) at user/example (REPL:1). ;; Assert failed: (or a b) (example {:a 1 :c 2}) ;; 1 nil ;; nil ;; check that the error-free average is between the max and min of the arguments: (defn avg [error & nums] {:post [(<= (apply min nums) % (apply max nums))]} (/ (apply + error nums) (count nums))) ;; #'user/avg (avg 0 1 2 3 4 5) ;; 3 (avg 100 1 2 3 4 5) ;; Execution error (AssertionError) at user/avg (REPL:1). ;; Assert failed: (<= (apply min nums) % (apply max nums))

by boraseoksoon on 2020-01-11 ▶

;; To inverse boolean (defn ! [bool] (if (= bool true) false true)) (! true) ;; => false (! false) ;; => true

by mdave16 on 2020-05-22 . May have evaluation errors. ▶

; You can also destructure inside the method (defn foo "Will be passed a hash" [{x :bar y :baz}] (+ x y)) (foo 1 2) ; Wrong number of args (2) passed (foo {:bar 1 :baz 2}) ; 3 (defn potentially-confusing-fn "Will be passed a hash with one key" [{x :match}] (* x x)) (potentially-confusing-fn 1) ; NullPointerException and nil return (potentially-confusing-fn {:match 2}) ; 4

See also

• clojure.core/def (4)
• clojure.core/defn- (1)
• clojure.core/defmacro (2)
• clojure.core/fn (3)
• clojure.core/declare (0)

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