Question

我想知道这样的列表是否可能：

FavoriteList [('d',False), ('e',True), ('c',False)]

简单地显示如下：

{'d', 'e'*, 'c'}

Answer 1

假设您正在谈论

favoriteList :: [(Char, Bool)]
favoriteList = [('d',False), ('e',True), ('c',False)]

你需要一个列表['d','e','c']（我希望*偶然到达那里）那就是

λ> map fst favoriteList
"dec"

（其中字符列表显示为字符串）。

假设您需要{...}格式，那么您必须多做一些工作。在Data.List.intersperse中使用", " 编织，在show添加' '以获得λ> import Data.List(intersperse) λ> "{" ++ (concat . intersperse ", " $ map (show . fst) favoriteList) ++ "}" "{'d', 'e', 'c'}"，您可以使用以下内容进行操作：

λ> map fst favoriteList
"dec"
λ> map (show . fst) favoriteList
["'d'","'e'","'c'"]
λ> intersperse ", " $ map (show . fst) favoriteList
["'d'",", ","'e'",", ","'c'"]
λ> concat . intersperse ", " $ map (show . fst) favoriteList
"'d', 'e', 'c'"

如果您对以下步骤感兴趣：

"{" ++ (..) ++ "}"

*应该是显而易见的，我希望

假设true在那里，因为元组的第二部分是λ> let stared (c,star) = if star then show c ++ "*" else show c λ> "{" ++ (concat . intersperse ", " $ map stared favoriteList) ++ "}" "{'d', 'e'*, 'c'}"，那么你可以这样做：

FavoriteList

进一步假设data FavoriteList = FavoriteList [(Char, Bool)] myList :: FavoriteList myList = FavoriteList [('d',False), ('e',True), ('c',False)]不是拼写错误而是数据构造函数：

λ> let stared (c,star) = if star then show c ++ "*" else show c
λ> let (FavoriteList xs) = myList in "{" ++ (concat . intersperse ", " $ map (stared) xs) ++ "}"
"{'d', 'e'*, 'c'}"

它的

import Data.List(intercalate)

data FavoriteList = FavoriteList [(Char, Bool)]

myList :: FavoriteList
myList = FavoriteList [('d',False), ('e',True), ('c',False)]

printList :: FavoriteList -> String
printList (FavoriteList xs) = 
  "{" ++ (intercalate ", " $ map stared xs) ++ "}"
  where  stared (c,star) = if star then show c ++ "*" else show c

我刚刚使用模式匹配来再次列出该列表

完整代码

intercalate = concat . intersperse

（ PS ：Chi指出我应该使用λ> printList myList "{'d', 'e'*, 'c'}"代替）示例

jspm install github:mygithubaccount/myplugin@master

*我希望这就是全部;）*

显示包含其他方面的列表

1 个答案:

完整代码