meta.md

name	display as	clojure doc	see also
syntax/meta	^ meta	http://clojure.org/reference/reader#_metadata	syntax/js-tag cljs.core/meta cljs.core/with-meta cljs.core/vary-meta cljs.core/alter-meta!

Summary

Attach metadata to the following symbol or collection. Use a map to specify the metadata, or use shorthand:

• ^:foo => ^{:foo true}
• ^"foo" => ^{:tag "foo"}
• ^foo => ^{:tag foo}

Details

Examples

Attach metadata to a collection:

^:foo [1 2 3]
;;=> [1 2 3]

View the resulting metadata:

(meta ^:foo [1 2 3])
;;=> {:foo true}

(meta ^{:foo "bar"} [1 2 3])
;;=> {:foo "bar"}

(meta ^"foo" [1 2 3])
;;=> {:tag "foo"}

(def foo 1)
(meta ^foo [1 2 3])
;;=> {:tag 1}

Chain metadata:

(meta ^:foo ^"foo" [1 2 3])
;;=> {:foo true, :tag "foo"}

Generally, metadata maps used purely by the compiler are not evaluated and should be read literally:

(defn foo [^js x]   ;; ^js => ^{:tag js}   (literal map not evaluated)
  ^js (.-bar x))    ;; ^js => ^{:tag js}   (literal map not evaluated)

But here’s one exception— metadata maps are evaluated when placed behind literal collections created at runtime:

(def foo (+ 1 2 3))
(meta ^foo {})                ;;=> {:tag 6}   not {:tag foo}
(meta ^{:foo (+ 1 2 3)} [])   ;;=> {:foo 6}   not {:foo (+ 1 2 3)}