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module Main where
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import Maybe (fromJust)
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import Arrow
 
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type Id = String
 
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data Val a = Num Int
	   | Bl Bool
	   | Fun (a (Val a) (Val a))
 
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data Exp = Var Id
	 | Add Exp Exp
	 | If Exp Exp Exp
	 | Lam Id Exp
	 | App Exp Exp
 
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eval ::
     (ArrowChoice a, ArrowApply a) => Exp -> a [(Id, Val a)] (Val a)
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eval (Var s) = (arr (\ env -> fromJust (lookup s env)))
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eval (Add e1 e2)
  = (arr (\ env -> (env, env)) >>>
       (first (eval e1) >>>
	  arr (\ (vP_0@ ~(Num u), env) -> (env, (u, vP_0))))
	 >>>
	 (first (eval e2) >>>
	    arr (\ (vP_1@ ~(Num v), (u, vP_0@ ~(Num u))) -> Num (u + v))))
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eval (If e1 e2 e3)
  = (arr (\ env -> (env, env)) >>>
       (first (eval e1) >>>
	  arr (\ (vP_0@ ~(Bl b), env) -> if b then Left env else Right env))
	 >>> (eval e2 ||| eval e3))
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eval (Lam x e)
  = (arr (\ env -> Fun ((arr (\ v -> (x, v) : env) >>> eval e))))
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eval (App e1 e2)
  = (arr (\ env -> (env, env)) >>>
       (first (eval e1) >>>
	  arr (\ (vP_0@ ~(Fun f), env) -> (env, (f, vP_0))))
	 >>>
	 (first (eval e2) >>> arr (\ (v, (f, vP_0@ ~(Fun f))) -> (f, v)))
	   >>> app)
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i = Lam "x" (Var "x")
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k = Lam "x" (Lam "y" (Var "x"))
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double = Lam "x" (Add (Var "x") (Var "x"))
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t = n
  where {-# LINE 35 "small/Eval.as" #-}
	Num n
	  = eval
	      (If (Var "b") (App (App k (App double (Var "x"))) (Var "x"))
		 (Add (Var "x") (Add (Var "x") (Var "x"))))
	      [("b", Bl True), ("x", Num 5)]