查找列表中的第一个重复元素

Question

我是Haskell的新手.我试图在Haskell中编写代码,从列表中找到第一个重复元素,如果它没有重复元素,则给出消息没有重复.我知道我可以通过nub功能来做到这一点,但我试图在没有它的情况下做到这一点.

Answer 1

这是一种方法:

import qualified Data.Set as Set

dup :: Ord a => [a] -> Maybe a
dup xs = dup' xs Set.empty
  where dup' [] _ = Nothing
        dup' (x:xs) s = if Set.member x s 
                           then Just x
                           else dup' xs (Set.insert x s)

dupString :: (Ord a, Show a) => [a] -> [Char]
dupString x = case dup x of
                  Just x  -> "First duplicate: " ++ (show x)
                  Nothing -> "No duplicates"

main :: IO ()
main = do
       putStrLn $ dupString [1,2,3,4,5]
       putStrLn $ dupString [1,2,1,2,3]
       putStrLn $ dupString "HELLO WORLD"

下面是它的工作原理:

*Main> main
No duplicates
First duplicate: 1
First duplicate: 'L'