clojure.core/with-meta

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Type: function Added: Clojure 1.0 Examples: 4 Runnable: 1

([obj m])

Returns an object of the same type and value as obj, with map m as its metadata.

Examples

by devn on 2010-08-02 ▶

(with-meta [1 2 3] {:my "meta"}) [1 2 3] (meta (with-meta [1 2 3] {:my "meta"})) {:my "meta"}

by mmwaikar on 2013-02-01 . May have evaluation errors. ▶

;; the same example above in a simplified way (def wm (with-meta [1 2 3] {:my "meta"})) #'user/wm wm [1 2 3] (meta wm) {:my "meta"}

by Rovanion on 2019-01-08 . May have evaluation errors. ▶

;; Trying to attach meta data to nil with throw an exception (with-meta nil {:my "meta"}) ;; Unhandled java.lang.NullPointerException ;; It also only works for Clojure data types implementing clojure.lang.IObj (require '[clojure.java.io :as io]) (with-meta (io/file ".") {:my "meta"}) ;; Unhandled java.lang.ClassCastException ;; java.io.File cannot be cast to clojure.lang.IObj

by jimka2001 on 2020-10-23 . May have evaluation errors. ▶

;; The meta data notation `^:dynamic` can be used to force a variable to be dynamic. (def ^:dynamic *my-dynamic-variable* "my doc string" the-initial-value) ;; However, using this notation within a macro does not have the same semantics. ;; For example the following macro *will not* expand to a definition of a ;; dynamic variable whose value is a function. (defmacro dyn-fun [name lambda-list & body] ;; WRONG `(def ^:dynamic ~name (fn ~lambda-list ~@body))) ;; The macro should be written something like the following. (defmacro dyn-fun [name lambda-list & body] `(def ~(with-meta name {:dynamic true}) (fn ~lambda-list ~@body)))

Note by beoliver

The Clojure documentation states that "metadata does not impact equality (or hash codes). Two objects that differ only in metadata are equal." One should be aware that while a function can be compared to itself:

(defn foo [x] (+ x x))

testing equality

> (= foo foo)
true
> (= (with-meta foo {:hello :world}) foo)
false
> (= (with-meta foo {}) foo)
false

See also

• clojure.core/vary-meta (1)
• clojure.core/meta (0)
• clojure.core/alter-meta!

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