CODEWITHSUNDEEP
clojure
ode
ode

Reputation: 63

Attaching Metadata in Clojure

I understand how to attach metadata such as:

(def x ^{:foo true} [1 2])

But a Clojure video on Youtube uses the example:

(def ^{:foo true} x [1 2])

(meta) doesn't return the metadata on the Youtube example.

What is it attaching the metadata to, why would you do this and how do I return the metadata? Thanks.

Upvotes: 3

Views: 202

Answers (1)

Kyle
Kyle

Reputation: 22258

^ is a reader macro that "first reads the metadata and attaches it to the next form"

Attach metadata to the vector [1 2]

(def x ^{:foo true} [1 2])
(meta x) ;; x resolves to the vector, `meta` retrieves the metadata attached to it

Attach metadata to the var x

(def ^{:foo true} x [1 2])
(meta #'x) ;; retrieve the metadata from the var `x`

Consider this repl session where we attach metadata to the var and the value:

user=> (def ^{:var-foo true} x ^{:val-foo true} [1 2])
#'user/x
user=> (meta x)
{:val-foo true}
user=> (meta #'x)
{:ns #<Namespace user>, :name x, :file "NO_SOURCE_PATH", :var-foo true, :column 1, :line 1}

Upvotes: 4

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