mathOp :: Parser (Expr -> Expr -> Expr)
mathOp = do
    op <- operator
    return $ case op of
                "+" -> addExpr
                "-" -> subExpr
                "*" -> mulExpr
                "/" -> divExpr
mathOp :: Parser (Expr -> Expr -> Expr)
mathOp = do
    op <- operator
    return $ case op of
                "+" -> addExpr
                "-" -> subExpr
                "*" -> mulExpr
                "/" -> divExpr
mathOp = do
    op <- operator
    return $ case op of
                "+" -> addExpr
                "-" -> subExpr
                "*" -> mulExpr
                "/" -> divExpr