extend-type

A macro that expands into an extend call. Useful when you are
supplying the definitions explicitly inline, extend-type
automatically creates the maps required by extend.  Propagates the
class as a type hint on the first argument of all fns.
 (extend-type MyType 
  Countable
    (cnt [c] ...)
  Foo
    (bar [x y] ...)
    (baz ([x] ...) ([x y & zs] ...)))
 expands into:
 (extend MyType
 Countable
   {:cnt (fn [c] ...)}
 Foo
   {:baz (fn ([x] ...) ([x y & zs] ...))
    :bar (fn [x y] ...)})