如何找到未包含在另一个haskell列表中的列表元素

Question

我是haskell的新手,想要在两个haskell列表之间设置区别.举例来说,我有列表a和b和希望a-b,即[4,5,6,7].

let a = [1,2,3,4,5,6,7] 
let b = [1,2,3]

是否有内置函数来执行此操作.

Answer 1

你可以使用(\\)运算符Data.List,警告它只删除每个运算符的第一个实例,所以

> [1..7] \\ [1..3]
[4,5,6,7]
> [1,1,1] \\ [1,2]
[1,1]

如果您想要真正的"设置差异",请使用Data.Set:

> import qualified Data.Set as Set
> Set.fromList [1..7] Set.\\ Set.fromList [1..3]
> Set.fromList [1,1,1]
fromList [1]
> Set.fromList [1,1,1] Set.\\ Set.fromList [1,2]
fromList []