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{-# LANGUAGE GADTs #-}
module Main where
data Term a where
Lit :: Int -> Term Int
IsZero :: Term Int -> Term Bool
If :: Term Bool -> Term a -> Term a -> Term a
Pr :: Term a -> Term b -> Term (a, b)
Fst :: Term (a, b) -> Term a
Snd :: Term (a, b) -> Term b
eval :: Term v -> v
eval (Lit n) = n
eval (IsZero t) = eval t == 0
eval (If t1 t2 t3) = if eval t1 then eval t2 else eval t3
eval (Pr t1 t2) = (eval t1, eval t2)
eval (Fst t) = case (eval t) of { (a,b) -> a }
eval (Snd t) = case (eval t) of { (a,b) -> b }
term = If (IsZero (Lit 1)) (Pr (Lit 2) (Lit 3)) (Pr (Lit 3) (Lit 4))
main = print (eval term)
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