module Expr where
= Op BinOp Expr Expr
| NumLit Int
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| ExpX Int
deriving (Eq, Show)
data BinOp = AddOp | MulOp
deriving (Eq, Show)
example1, example2, example3, example4, example5 :: Expr
example1 = Op AddOp (NumLit 1) (Op AddOp (NumLit 2) (NumLit 3))
example2 = Op AddOp (NumLit 1) (Op MulOp (NumLit 2) (NumLit 3))
example3 = Op MulOp (NumLit 1) (Op AddOp (NumLit 2) (NumLit 3))
example4 = ExpX 4
example5 = Op MulOp (NumLit 1) (Op AddOp (ExpX 4) (NumLit 3))
eval :: Int -> Expr -> Int
eval x (ExpX n) = x^n
eval _ (NumLit y) = y
eval x (Op AddOp a b) = eval x a + eval x b
eval x (Op MulOp a b) = eval x a * eval x b
prettyExpr :: Expr -> String
prettyExpr (NumLit x) = show x
prettyExpr (ExpX 1) = “x”
prettyExpr (ExpX n) = “x^” ++ show n
prettyExpr (Op AddOp x y) = prettyExpr x ++ ” + ” ++ prettyExpr y
prettyExpr (Op MulOp AddOp x1 x2) AddOp y1 y2))
= concat [“(“, prettyExpr x, “) * (“, prettyExpr y, “)”]
prettyExpr (Op MulOp AddOp x1 x2) y)
= concat [“(“, prettyExpr x, “) * “, prettyExpr y]
prettyExpr (Op MulOp x AddOp y1 y2))
= concat [prettyExpr x, ” * (“, prettyExpr y, “)”]
prettyExpr (Op MulOp x y)
= concat [prettyExpr x, ” * “, prettyExpr y]
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