如何从haskell中的元组中提取元素

Question

我有以下元组：

type Id = Int
type Name = String
type FSK = Int
type Movie = (Id, Name, FSK)

现在我想定义一个从给定列表中提取电影的功能：

extract :: Id -> [Movie] -> (Maybe Movie, [Movie])
extract = .... ?? 

所以当我给出一个id和一个电影列表时，它会提取： 1）如果找到id，则没有+给定列表 2）只是+电影，没有那部电影的新列表，如果找到给定的id，并且电影从列表中删除

示例：

*Main> extract 0 [(1,"Matrix",16),(2,"Gladiator",0)]
(Nothing,[(1,"Matrix",16),(2,"Gladiator",0)])
*Main> extract 1 [(1,"Matrix",16),(2,"Gladiator",0)]
(Just (1,"Matrix",16),[(2,"Gladiator",0)])

我该如何定义这个功能？

Answer 1

extract :: Id -> [Movie] -> (Maybe Movie, [Movie])
extract id []                 = (Nothing, [])
extract id ((fid, n, f):list) | fid == id =  (Just (fid, n, f), list)
                              | otherwise =  (found, (fid, n, f):tail)
                              where (found, tail) = extract id list