我如何使用Control.Lens更新列表的第i个元素？ [英] How can I use Control.Lens to update the ith element of a list?

问题描述

  data数据Outer = Outer {_list :: [Inner]} 
数据Inner = Inner {_bool :: Bool} 
  

使用Control.Lens，我可以访问_bool （这是一个'State Outer'monad内部）像这样

  boolValue<  -  gets（^。list。to （!! i）。inner）
  

我也希望能够用一些东西来更新这个值像

  list ^。 （to（!! i））^。内心％=真
  

然而（据我了解），'to'函数只创建一个getter，不是一个真正的镜头，可以用作吸气剂或吸气剂。

那么，我该如何将（!! i）转换成一个镜头来让我更新这个领域？

解决方案

你不能将（!!）变成除 Getter - 但有一个函数可以做这样的事情： ix ，以访问索引处的内容。它实际上是一个遍历，而不是 Lens - 在这里，这意味着它可能失败（如果索引是超出界限的） - 但只要索引在列表中，它就可以工作。

还有一个问题，即 - （^。）也是专门用于获取值的运算符。它与例如不兼容（％=），它将镜头般的东西当作第一个参数。和：（％=）用于将函数映射到现有值;如果你只是想设置，你可以使用（。=）。所以你可能想要这样的东西：

  list。 ix i。 inner。= True 
  

*实际上有一个函数可以做到这一点 - 它被称为在 - 但它使用美妙的邪恶黑魔法，你不应该使用它，至少不是用于这个（也可能不适用于任何真正的代码）。

I have some datatypes along the line of

data Outer = Outer { _list :: [ Inner ] }
data Inner = Inner { _bool :: Bool }

using Control.Lens, I can access the _bool of the ith Inner (inside a 'State Outer' monad) like this

boolValue <- gets (^. list . to (!! i) . inner)

I would like to also be able to update this value with something like

list ^. (to (!! i)) ^. inner %= True

However (by my understanding), the 'to' function only creates a getter, not a true lens that can be used as either getter or setter.

So, how can I convert (!! i) into a lens that will allow me to update this field?

解决方案

You can't* turn (!!) into any lens-like thing other than a Getter -- but there's a function to do this sort of thing: ix, for accessing things at indices. It's actually a Traversal, not a Lens -- which, here, just means that it can fail (if the index is out of bounds) -- but as long as the index is in the list, it'll work.

There's another problem, though -- (^.) is also an operator that's used exclusively for getting values. It's incompatible with e.g. (%=), which takes a lens-like thing as its first argument. And: (%=) is for mapping a function over the existing value; if you just want to set, you can use (.=). So you probably want something like:

list . ix i . inner .= True

* There actually is a function that can do this -- it's called upon -- but it uses wonderful evil black magic and you shouldn't use it, at least not for this (and probably not for any real code).

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