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+module Minilang.Lambda.EvalSpec (spec) where
+
+import Control.Concurrent (threadDelay)
+import Control.Concurrent.Async (race_)
+import Minilang.Lambda.Eval (Term (..), eval, evalNeed)
+import Test.Hspec (Spec, describe, it, shouldBe)
+
+spec :: Spec
+spec = do
+  let fix = Lam "x" (App (Var "x") (Var "x"))
+  let omega = App fix fix
+  let term = App (App (Lam "x" (Lam "y" (Var "y"))) omega) (Var "z")
+
+  describe "call-by-value eval" $ do
+    it "evaluates 'x' as itself" $ do
+      let term = Var "x"
+      eval term [] `shouldBe` Var "x"
+    it "evaluates '((λ x. x) y)' as 'y'" $ do
+      let term = App (Lam "x" (Var "x")) (Var "y")
+      eval term [] `shouldBe` Var "y"
+    it "evaluates '((λ x y. y) y z t)' as '(z t)'" $ do
+      let snd' = Lam "x" (Lam "y" (Var "y"))
+      let term = App (App (App snd' (Var "y")) (Var "z")) (Var "t")
+      eval term [] `shouldBe` App (Var "z") (Var "t")
+    it "diverges on evaluating argument bound to ω" $
+      shouldDiverge $ eval term []
+
+  describe "call-by-need eval" $ do
+    it "evaluates 'x' as itself" $ do
+      evalNeed (Var "x") [] `shouldBe` Var "x"
+    it "evaluates '(λ x y . y) ⊥ z' as 'z'" $ do
+      evalNeed term [] `shouldBe` Var "z"
+    it "diverges on evaluating ω" $
+      shouldDiverge $ evalNeed omega []
+
+shouldDiverge :: Term -> IO ()
+shouldDiverge t =
+  race_
+    (t `seq` fail "Did not diverge")
+    (threadDelay 1000000)