如果我声明数据构造函数,例如
data City = Baltimore | Chicago | Seattle | Miami | Toronto
deriving (Bounded, Enum, Eq, Ord, Show)
data Name = Orioles | Cubs | Mariners | Marlins | BlueJays
deriving (Bounded, Enum, Eq, Ord, Show)
如何创建一个函数
checkPermutation :: (City -> Name) -> Bool
检查没有为两个城市分配相同的团队名称。例如,以下内容将返回 True,但如果将任何“名称”分配给多个城市,则会返回 False。
test1 :: City -> Name
test1 c = case c of
Baltimore -> Orioles
Chicago -> Cubs
Seattle -> Mariners
Miami -> Marlins
Toronto -> Blue Jays
最佳答案
试试这个:
import Data.List (nub)
cities :: [City]
cities = [Baltimore..Toronto]
checkPermutation :: (City -> Name) -> Bool
checkPermutation f = (== length cities) . length . nub . map f $ cities
这主要检查函数f::City -> Name
是否为injective .
事实上,我们可以创建一个更通用的单射
谓词:
import Data.Set as Set
typeSet :: (Bounded a, Enum a, Ord a) => Set a
typeSet = fromList $ enumFrom minBound
injective :: (Enum a, Bounded a, Ord a, Ord b) => (a -> b) -> Bool
injective f = let xs = typeSet in (== size xs) . size . Set.map f $ xs
希望有帮助。
关于function - 检查排列的 Haskell 函数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/32663739/