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(.module:
  [lux #*
   ["_" test (#+ Test)]
   [abstract
    [equivalence (#+ Equivalence)]
    [monad (#+ do)]]
   [control
    ["." function]]
   [math
    ["." random]
    [number
     ["n" nat]]]]
  [\\
   ["." / (#+ Functor)]])

(type: #export (Injection f)
  (All [a] (-> a (f a))))

(type: #export (Comparison f)
  (All [a]
    (-> (Equivalence a)
        (Equivalence (f a)))))

(def: (identity injection comparison (^open "@//."))
  (All [f] (-> (Injection f) (Comparison f) (Functor f) Test))
  (do {! random.monad}
    [sample (\ ! map injection random.nat)]
    (_.test "Identity."
            ((comparison n.=)
             (@//map function.identity sample)
             sample))))

(def: (homomorphism injection comparison (^open "@//."))
  (All [f] (-> (Injection f) (Comparison f) (Functor f) Test))
  (do {! random.monad}
    [sample random.nat
     increase (\ ! map n.+ random.nat)]
    (_.test "Homomorphism."
            ((comparison n.=)
             (@//map increase (injection sample))
             (injection (increase sample))))))

(def: (composition injection comparison (^open "@//."))
  (All [f] (-> (Injection f) (Comparison f) (Functor f) Test))
  (do {! random.monad}
    [sample (\ ! map injection random.nat)
     increase (\ ! map n.+ random.nat)
     decrease (\ ! map n.- random.nat)]
    (_.test "Composition."
            ((comparison n.=)
             (|> sample (@//map increase) (@//map decrease))
             (|> sample (@//map (|>> increase decrease)))))))

(def: #export (spec injection comparison functor)
  (All [f] (-> (Injection f) (Comparison f) (Functor f) Test))
  (<| (_.for [/.Functor])
      ($_ _.and
          (..identity injection comparison functor)
          (..homomorphism injection comparison functor)
          (..composition injection comparison functor)
          )))