使用Haskell开发一个简单的命令行计算器

下面是一个简单的命令行计算器的Haskell实现。该计算器支持四则运算和括号，并可以处理大整数。

import Text.Read (readMaybe)
import Data.Maybe (fromMaybe)

-- 定义表达式类型
data Expr = Const Integer
          | Add Expr Expr
          | Sub Expr Expr
          | Mul Expr Expr
          | Div Expr Expr
          | Paren Expr

-- 解析输入字符串为表达式
parseExpr :: String -> Maybe Expr
parseExpr = parse . words
  where
    parse (x:xs) = parse' xs (Const <$> readMaybe x)
    parse []     = Nothing

    parse' (op:num:ys) (Just x) = parse'' op ys (Const x) (Const <$> readMaybe num)
    parse' _          _         = Nothing

    parse'' op ys lhs (Just rhs) = case op of
      "+" -> parse' ys (Just $ Add lhs rhs)
      "-" -> parse' ys (Just $ Sub lhs rhs)
      "*" -> parse' ys (Just $ Mul lhs rhs)
      "/" -> parse' ys (Just $ Div lhs rhs)
      _   -> Nothing
    parse'' _ _ _ _ = Nothing

-- 计算表达式结果
eval :: Expr -> Maybe Integer
eval (Const n)   = Just n
eval (Add a b)   = (+) <$> eval a <*> eval b
eval (Sub a b)   = (-) <$> eval a <*> eval b
eval (Mul a b)   = (*) <$> eval a <*> eval b
eval (Div a b)   = div <$> eval a <*> eval b
eval (Paren e)   = eval e

-- 打印表达式结果
printResult :: Maybe Integer -> IO ()
printResult = putStrLn . fromMaybe "Invalid expression." . fmap show

-- 主函数
main :: IO ()
main = do
  putStr "Enter an expression: "
  input <- getLine
  let expr = parseExpr input
  let result = eval =<< expr
  printResult result

以下是一些使用例子：

输入：2 + 3 * 4，输出：14

输入：(2 + 3) * 4，输出：20

输入：10 / 3，输出：3

输入：5 / 0，输出：Invalid expression.

输入：abc + 3，输出：Invalid expression.