Haskell中如何修改Show与formatList函数生成指定Trip输出格式

原始代码 #

type Start = String
type Destination = String
type Distance = Float         -- km
type AverageSpeed = Float   -- m/s 
type FirstClass = Bool

data Trip = Trip Start [TravelOption]
data TravelOption = Plane AverageSpeed Destination Distance FirstClass | Bus AverageSpeed Destination Distance | Train AverageSpeed Destination Distance FirstClass

class VacationTrip a 
  where 

instance Show TravelOption
where
show (Plane speed dest distance firstClass) = " by plane to " ++ dest
show (Bus speed dest distance) = " by bus to " ++ dest
show (Train speed dest distance firstClass) = "by train to " ++ dest

instance Show Trip
where
show (Trip start travelOptions) = "[" ++ start ++ "]" ++ formatList (map show travelOptions)
where
formatList :: [String] -> String
formatList [] = ""
formatList [x] = "[" ++ x ++ "]"
formatList (x:xs) = "(" ++ x ++ ") " ++ formatList xs

问题说明 #

当前代码生成的输出格式为：[Amsterdam] (by plane to Berlin) (by train to Paris) [by train to Cologne]，但需要的输出格式为：[起点] by plane to (下一个城市) by train to (下一个城市) by train to [最后一个城市]，要求每个中途城市用括号包裹，最后一个城市用方括号包裹。

修改方案 #

通过拆分交通描述与目的地的处理逻辑，调整Trip的Show实例实现需求格式：

type Start = String
type Destination = String
type Distance = Float         -- km
type AverageSpeed = Float   -- m/s 
type FirstClass = Bool

data Trip = Trip Start [TravelOption]
data TravelOption = Plane AverageSpeed Destination Distance FirstClass | Bus AverageSpeed Destination Distance | Train AverageSpeed Destination Distance FirstClass

class VacationTrip a 
  where 

-- 提取交通方式的描述文本
getTravelDesc :: TravelOption -> String
getTravelDesc (Plane _ _ _ _) = " by plane to"
getTravelDesc (Bus _ _ _) = " by bus to"
getTravelDesc (Train _ _ _ _) = " by train to"

-- 提取旅行选项的目的地
getDestination :: TravelOption -> Destination
getDestination (Plane _ dest _ _) = dest
getDestination (Bus _ dest _) = dest
getDestination (Train _ dest _ _) = dest

instance Show Trip where
  -- 处理空行程的边界情况
  show (Trip start []) = "[" ++ start ++ "]"
  show (Trip start opts) = 
    "[" ++ start ++ "]" 
    ++ concatMap (\(desc, dest) -> desc ++ " (" ++ dest ++ ")") initPairs 
    ++ lastDesc ++ " [" ++ lastDest ++ "]"
    where
      -- 将所有旅行选项拆分为(交通描述, 目的地)的配对
      allPairs = map (\opt -> (getTravelDesc opt, getDestination opt)) opts
      -- 取除最后一个外的所有配对
      initPairs = init allPairs
      -- 提取最后一个选项的描述和目的地
      (lastDesc, lastDest) = last allPairs

修改说明 #

1. 新增getTravelDesc和getDestination两个辅助函数，分别提取每个TravelOption的交通方式描述和目的地，让格式控制更灵活。
2. 重构Trip的Show实例：
   • 增加空行程的边界处理，避免列表操作报错。
   • 将所有旅行选项转换为(交通描述, 目的地)的配对列表。
   • 对中途的每个配对，拼接为交通描述 (目的地)的格式。
   • 最后一个配对单独处理为交通描述 [目的地]的格式。
   • 组合起点、中途部分和最后部分，生成符合要求的输出。

测试示例：

testTrip = Trip "Amsterdam" [Plane 250 "Berlin" 650 True, Train 150 "Paris" 800 False, Train 120 "Cologne" 500 True]

输出结果：[Amsterdam] by plane to (Berlin) by train to (Paris) by train to [Cologne]

内容的提问来源于stack exchange，提问作者Adnan Eren Güngörmüş