Available since 1.2 (source)
  • (extend atype & proto+mmaps)
Implementations of protocol methods can be provided using the extend construct:
 (extend AType
   {:foo an-existing-fn
    :bar (fn [a b] ...)
    :baz (fn ([a]...) ([a b] ...)...)}
  extend takes a type/class (or interface, see below), and one or more
protocol + method map pairs. It will extend the polymorphism of the
protocol's methods to call the supplied methods when an AType is
provided as the first argument. 
 Method maps are maps of the keyword-ized method names to ordinary
fns. This facilitates easy reuse of existing fns and fn maps, for
code reuse/mixins without derivation or composition. You can extend
an interface to a protocol. This is primarily to facilitate interop
with the host (e.g. Java) but opens the door to incidental multiple
inheritance of implementation since a class can inherit from more
than one interface, both of which extend the protocol. It is TBD how
to specify which impl to use. You can extend a protocol on nil.
 If you are supplying the definitions explicitly (i.e. not reusing
exsting functions or mixin maps), you may find it more convenient to
use the extend-type or extend-protocol macros.
 Note that multiple independent extend clauses can exist for the same
type, not all protocols need be defined in a single extend call.
 See also:
extends?, satisfies?, extenders