Haskell如何评估撤消列表插入的功能？

Question

我试图了解Haskell如何评估sep [1, 2, 3, 4, 5]以获得([1, 3], [2, 4, 5]) ，其中：

sep [ ] = ([ ], [ ])
sep [x] = ([ ], [x])
sep (x1:x2:xs) = let (is, ps) = sep xs in (x1:is, x2:ps)

我开始是这样的：

sep [1, 2, 3, 4, 5] = let (is, ps) = sep [3, 4, 5] in (1:is, 2:ps)

但是之后？

Answer 1

终于我明白了。

1) sep [1, 2, 3, 4, 5] = let (is, ps) = sep [3, 4, 5] in (1:is, 2:ps)

2) sep [3, 4, 5] = let (is, ps) = sep [5] in (3:is, 4:ps)

3) sep [5] = ([], [5])

在2）中sep [3, 4, 5] = let (is, ps) = ([], [5]) in (3:is, 4:ps) = ([3], [4, 5])

在1）中sep [1, 2, 3, 4, 5] = let (is, ps) = ([3], [4, 5]) in (1:is, 2:ps) = ([1, 3], [2, 4, 5])