使用过滤器过滤

给出一个清单：

 li = [1,2,3,4,5]

我们可以使用 filter :: (a -> Bool) -> [a] -> [a] 过滤带谓词的列表：

 filter (== 1) li       -- [1]
 
 filter (even) li       -- [2,4]
 
 filter (odd) li        -- [1,3,5]
 
 -- Something slightly more complicated
 comfy i = notTooLarge && isEven
   where 
      notTooLarge = (i + 1) < 5
      isEven = even i
 
 filter comfy li        -- [2]

当然，这不只是关于数字：

 data Gender = Male | Female deriving Show
 data Person = Person String Gender Int deriving Show
 
 onlyLadies :: [Person] -> Person
 onlyLadies x = filter isFemale x
   where 
     isFemale (Person _ Female _) = True
     isFemale _ = False
 
 onlyLadies [(Person "Alex" Male 31),(Person "Ellie" Female 29)]
 -- [Person "Ellie" Female 29]