Available since 1.0 (source)
  • (deref ref)
  • (deref ref timeout-ms timeout-val)
Also reader macro: @ref/@agent/@var/@atom/@delay/@future/@promise. Within a transaction,
returns the in-transaction-value of ref, else returns the
most-recently-committed value of ref. When applied to a var, agent
or atom, returns its current state. When applied to a delay, forces
it if not already forced. When applied to a future, will block if
computation not complete. When applied to a promise, will block
until a value is delivered.  The variant taking a timeout can be
used for blocking references (futures and promises), and will return
timeout-val if the timeout (in milliseconds) is reached before a
value is available. See also - realized?.