clojure.core/declare

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

([& names])

defs the supplied var names with no bindings, useful for making forward declarations.

Examples

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

(defn foo [] (undefined-func)) ; Evaluation aborted. Unable to resolve symbol: undefined-func in this context nil (declare undefined-func) #'user/undefined-func (defn foo [] (undefined-func)) #'user/foo

by lu4nx on 2013-12-02 . May have evaluation errors. ▶

(declare show) #'user/show (defn welcome [user-name] (prn (show) user-name)) #'user/welcome (defn show [] (prn "welcome ")) #'user/show (welcome "lu4nx") "welcome " nil "lu4nx" nil

by kimtg on 2015-03-07 . May have evaluation errors. ▶

; def will do too. (def show) #'user/show (defn welcome [user-name] (prn (show) user-name)) #'user/welcome (defn show [] (prn "welcome")) #'user/show (welcome "lu4nx") "welcome" nil "lu4nx" nil

by owenRiddy on 2017-08-12 . May have evaluation errors. ▶

;;Declare (like "def") has a natural partner in "var". ;;Consider trying to embed a to-be-defined function in a data structure: (declare foo) (def bar {:handy-fn foo}) (defn foo [] 42) ((:handy-fn bar)) ;;IllegalStateException Attempting to call unbound fn: #'user/foo clojure.lang.Var$Unbound.throwArity (Var.java:43) ;;:handy-fn in bar is now permanently linked to the unbound var ;; present when the def was evaluated. This can be avoided ;; by not evaluating foo when creating bar. (declare foo) ;;(def bar {:handy-fn (var foo)}) (def bar {:handy-fn #'foo}) (defn foo [] 42) ((:handy-fn bar)) ;; 42

See also

• clojure.core/def (4)
• clojure.core/letfn (1)

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