clojure.core/doto

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Type: macro Added: Clojure 1.0 Examples: 4 Runnable: 0

([x & forms])

Evaluates x then calls all of the methods and functions with the value of x supplied at the front of the given arguments. The forms are evaluated in order. Returns x.

(doto (new java.util.HashMap) (.put "a" 1) (.put "b" 2))

Examples

by dakrone on 2010-12-30 . May have evaluation errors. ▶

;; Note that even though println returns nil, doto still returns the HashMap object (doto (java.util.HashMap.) (.put "a" 1) (.put "b" 2) (println)) ;;=> #<HashMap {b=2, a=1}> ;;=> {"b" 2, "a" 1} ;; Equivalent to (def m (java.util.HashMap.)) (.put m "a" 1) (.put m "b" 2) m ;;=> {"a" 1, "b" 2} (println m) ;;=> #object[java.util.HashMap 0x727fcc37 {a=1, b=2}]

by shockbob on 2011-08-21 . May have evaluation errors. ▶

;; quick demonstration of using a Collections function on the resulting ArrayList (def al (doto (java.util.ArrayList.) (.add 11) (.add 3) (.add 7))) #'user/al al #<ArrayList [11, 3, 7]> (java.util.Collections/sort al) nil al #<ArrayList [3, 7, 11]>

by jimpil on 2012-06-15 . May have evaluation errors. ▶

;; careful when calling 'dotimes' from within a 'doto' statement (doto (java.util.ArrayList.) (.add -2) (.add -1) (dotimes [i 3] (.add i))) java.lang.IllegalArgumentException: dotimes requires a vector for its binding (NO_SOURCE_FILE:1) ; what has happened is that (java.util.ArrayList.) has secretly ; become the first argument to 'dotimes' and thus the exception ; informs us that it can't find the binding vector required for ; 'dotimes' to expand. You can cure this behaviour by simply using ; 'do' instead of 'doto' or by wrapping the call to 'dotimes' in ; a function. e.g: ;using 'let' with implicit 'do' instead of 'doto' (let [al (java.util.ArrayList.)] (.add al -2) (.add al -1) (dotimes [i 3] (.add al i)) al);return the ArrayList #<ArrayList [-2, -1, 0, 1, 2]> ;exactly what we intended ;wrapping 'dotimes' in a function literal (doto (java.util.ArrayList.) (.add -2) (.add -1) (#(dotimes [i 3] (.add % i)))) #<ArrayList [-2, -1, 0, 1, 2]> ;exactly what we intended again

by benjamin-asdf on 2021-10-08 . May have evaluation errors. ▶

; useful during development when you want to log something ; without changing the structure of your code (+ (doto 42 println) 10) => 52 ; similarly wrapping tap> in a threading macro for intermediate results (-> :hello {:hello "Hello"} (doto tap>) ; tap> "Hello" (str " World!")) => "Hello World!"

See also

• clojure.core/-> (4)
• clojure.core/->> (7)
• clojure.core/.. (3)

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