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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
eval :: Term a -> a
eval (Lit n) = n
eval (IsZero t) = eval t == 0
eval (If t1 t2 t3) = if eval t1 then eval t2 else eval t3
term = If (IsZero (Lit 1)) (Lit 2) (Lit 3)
main = print (eval term)
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