#<> unreadable

A proper definition for something that cannot be read.

• #<...>

When certain values cannot be printed to a REPL using some literal syntax form, it wraps a description of its value in a form defined as unreadable, #<>. A reader error will be thrown if this value is fed back into the REPL.

For example, (atom 42) will print #<Atom: 42> when evaluated in a REPL. This communicates a human-readable form that is not intended to be reproduce its value.

Oftentimes, evaluating JavaScript objects in a REPL will print the result of its .toString method inside the unreadable form #<>.

Unreadable forms will throw an exception when read:

#
;; clojure.lang.ExceptionInfo: Unreadable form

You can create an unreadable form for a custom type:

(deftype Foo [])
(Foo.)
;;=> #<[object Object]>

(deftype Foo [x]
  Object
  (toString [_]
    (str "Foo: " x)))
(Foo. 1)
;;=> #

Some examples of unreadable JavaScript values:

Math/sin
;;=> #

js/console
;;=> #<[object Object]>

Reader table @ tools.reader:src/main/clojure/clojure/tools/reader.clj

(defn- dispatch-macros [ch]
  (case ch
    \^ read-meta                ;deprecated
    \' (wrapping-reader 'var)
    \( read-fn
    \= read-eval
    \{ read-set
    \< (throwing-reader "Unreadable form")
    \" read-regex
    \! read-comment
    \_ read-discard
    \? read-cond
    \: read-namespaced-map
    \# read-symbolic-value
    nil))