没有重复构造函数的adt集合

Question

如果我有一个数据类型:

data Component = PositionC Double Double | HealthC Double | NameC String

• 我想要Component s的集合
• 我不会存储任何副本(即:它不会有两个healthc)
我不想手动复制代码

我可以使用什么技术、数据结构、TemplateHaskell特性等来满足上述标准?

我尝试过数据。设置和数据。映射失败

数据。地图的尝试:

import Data.Map
data Component = PositionC Double Double | HealthC Double | NameC String
-- Unacceptable code duplication
data ComponentType = PositionCT | HealthCT | NameCT
    deriving (Eq, Ord)
type ComponentMap = Map ComponentType Component
foo :: ComponentMap
-- More annoying code duplication
foo = fromList [(HealthCT, HealthC 100), (NameCT, NameC "John")] 
fooHealth = foo ! HealthCT -- However, accessing works great

数据。尝试:

import Data.Set (Set)
import qualified Data.Set as Set
data Component = PositionC Double Double | HealthC Double | NameC String
    deriving Eq
foo :: Set Component
foo = Set.fromAscList [HealthC 100, NameC "John"] -- This works well
-- But accessing elements is very bad
fooHealth = Set.elemAt 0 $ Set.filter p foo
    where p (HealthC _) = True
          p _           = False

Answer 1

一个选项是使用带有可选字段的记录:

data Component = Component 
                 { positionC :: Maybe (Double, Double)
                 ; healthC :: Maybe Double 
                 ; nameC :: Maybe String
                 }

直观地说，访问字段也比使用Set或Map便宜。

设置字段

moveToOrigin c = c { positionC = Just (0,0) }

获取字段

-- Requires health to be present
isDead c = fromJust (healthC c) == 0
-- If health is unknown returns False
saferIsDead c = health C == Just 0