让我们看看该函数对示例输入执行的操作:
transpose [[1,2,3],[4,5,6],[7,8,9]]
<=>
(map head [[1,2,3],[4,5,6],[7,8,9]]) : (transpose (map tail [[1,2,3],[4,5,6],[7,8,9]]))
<=>
[1,4,7] : (transpose [[2,3],[5,6],[8,9]])
<=>
[1,4,7] : (map head [[2,3],[5,6],[8,9]]) : (transpose (map tail [[2,3],[5,6],[8,9]]))
<=>
[1,4,7] : [2,5,8] : (transpose [[3],[6],[9]])
<=>
[1,4,7] : [2,5,8] : (map head [[3],[6],[9]]) : (transpose (map tail [[3],[6],[9]]))
<=>
[1,4,7] : [2,5,8] : [3, 6, 9] : (transpose [[], [], []])
<=>
[1,4,7] : [2,5,8] : [3, 6, 9] : [] -- because transpose ([]:_) = []
<=>
[[1,4,7],[2,5,8],[3,6,9]]
请注意,我选择减少项的顺序与 haskell 将使用的评估顺序不同,但这不会改变结果。
编辑:回应您编辑的问题:
is this
(map head x)
创建每个列表的头元素的列表?
是的。