haskell - 函数所有可能值的列表

Question

我有多个对象，每个对象都与一个字符串相关联。是否有某种模式允许以类型安全的方式获取所有字符串的列表？

data MyObject = Foo | Bar 

getObjectString :: MyObject -> String
getObjectString Foo = "Foo"
getObjectString Bar = "Bar"

-- Bad because not type safe and String duplication
listOfAllObjectStrings :: [String]
listOfAllObjectStrings = ["Foo", "Bar"] 

仍然不是类型安全但减少字符串重复的不同解决方案。

data MyObject = Foo | Bar 

getObjectString :: MyObject -> String
getObjectString Foo = listOfAllObjectString !! 0
getObjectString Bar = listOfAllObjectString !! 1

listOfAllObjectStrings :: [String]
listOfAllObjectStrings = ["Foo", "Bar"]

Answer 1

一旦你让它派生Enum和Bounded，你就可以获得MyObject的所有值。然后，您将对每个值应用 getObjectString 以获取字符串值。

data MyObject = Foo | Bar deriving (Enum, Bounded)

getObjectString :: MyObject -> String
getObjectString Foo = "Foo"
getObjectString Bar = "Bar"

listOfAllObjectStrings :: [String]
listOfAllObjectStrings = map getObjectString [minBound .. maxBound]