haskell中元素的索引

Question

我有这两个列表[@,a,@,b,c,@,@,@,(de),@,@,@,f,g,@,h]和["","","+","","","?","|","","","","","*","","","|",""]，我想通过使用scanl or map函数来覆盖第二个列表，并在第一个列表中提取它前面的元素。这两个列表有不同类型的元素。

例如，我想如果第二个列表中的元素是/= ""，那么在第一个列表中提取其后面的元素（例如，+获取a，{ {1}}，? -> c *->(de)）。

我试过这样的事情

|->g

其中map(\x-> if x /= "" then l2 !! (((elemIndices x l1)!!0)-1) else Epsilon)l1 和l1 is ["","","+","","","?","|","","","","","*","","","|",""]但不起作用，l2 is [@,a,@,b,c,@,@,@,(de),@,@,@,f,g,@,h]也来自@ = Epsilon。

有人可以帮助我吗？

Answer 1

我想你想要像

这样的东西

combinedList = zip secondList (Epsilon : firstList)

然后查找“？”你只需要lookup "?" combinedList并获得Just (Literal 'c')。

（显然你可以尝试抓住索引，但这里描述的情况似乎有点迂回。）