(doc Unit "is the empty type, also represented as `()` and equivalent to `void`
in C.")
(defmodule Unit
  (implements prn prn)
  (sig prn (Fn [Unit] String))
  (defn prn [unit]
    @"()")

  (doc copy 
    "'copies' a reference to a Unit value."
    "This function just returns a fresh value of type Unit.")
  (implements copy copy)
  (sig copy (Fn [(Ref Unit)] Unit))
  (defn copy [unit-ref]
    ())

  (doc zero 
    "Returns a fresh value of type Unit (this value performs no side-effects).")
  (implements zero zero)
  (sig zero (Fn [] Unit))
  (defn zero []
    ())

  (implements = =)
  (sig = (Fn [Unit Unit] Bool))
  (defn = [unit-a unit-b]
    true)
)

(defmodule UnitRef
  (implements = =)
  (sig = (Fn [&Unit &Unit] Bool))
  (defn = [unit-a unit-b]
    true)
)