如何在列表理解中使用过滤器？

Question

所以我试图从2个集合列表中创建一个setSubtraction，它返回xs中但不是ys中出现的所有元素的集合。例如xs = [1,2,3]和ys = [3,4,5]，则setSubtraction xs ys = [1,2]

这是我到目前为止所拥有的

setSubtract xs ys = [x | x <- xs, y <- ys, filter (==y) xs]

但过滤功能似乎错误

Answer 1

我稍微更改了您的代码，并过滤(== x) ys == []检查x未显示ys。

这是定义：

setSubtract xs ys = [x | x <- xs, filter ( == x) ys == []]

这是在行动：

λ>  setSubtract [1,2,3] [3,4,5]
[1,2]
λ> setSubtract [1,2,3,4,5,6,7] [3,4,5]
[1,2,6,7]
λ> 

替代定义

setSubtract xs ys = [x | x <- xs, not . any ( == x) $ ys]

Answer 2

 setSubtract cmp xs ys = [x | x <- xs, member cmp x ys == False]