用`到`镜头踩到拉链

Question

我正在努力使用镜头和拉链.考虑下面运行的代码ghci

> import Control.Lens
> import Control.Zipper
> 
> :t within (ix 1) $ zipper ([1,2,3] :: [Int]) 
> within (ix 1) $ zipper ([1,2,3] :: [Int])
   :: Control.Monad.MonadPlus m => m (Zipper Top Int [Int] :>> Int)

有data A t = A t,我怎样才能创建拉链类型:Control.Monad.MonadPlus m => m (Zipper Top Int [Int] :>> A Int)？

我试过within (ix 1 . to A) $ zipper ([1,2,3] :: [Int])但是它给出了一个错误:

Could not deduce (Contravariant
                    (Bazaar (Indexed Int) (A Int) (A Int)))
  arising from a use of ‘to’
from the context (Control.Monad.MonadPlus m)
  bound by the inferred type of
           it :: Control.Monad.MonadPlus m =>
                 m (Zipper Top Int [Int] :>> A Int)
  at Top level
In the second argument of ‘(.)’, namely ‘to A’
In the first argument of ‘within’, namely ‘(ix 1 . to A)’
In the expression: within (ix 1 . to A)

Answer 1

一种方法是制作一个Iso并用它进行创作。在 ghci 中：

> import Control.Lens
> import Control.Zipper
>
> data A t = A t
> let _A = iso A (\(A a) -> a)
>
> let a = within (ix 1 . _A) $ zipper ([1,2,3] :: [Int])
> :t a
a :: MonadPlus m => m (Zipper Top Int [Int] :>> A Int)
> a ^? _Just . focus
Just (A 2)

编辑：您需要的原因(\(A a) -> a)是这样您就可以退出。

> data A t = A t
> let _A = iso A (error "Can't unA")
>
> let a = within (ix 1 . _A) $ zipper ([1,2,3] :: [Int])
> a ^? _Just . focus
Just (A 2)
> fmap upward a ^? _Just . focus
Just [1,*** Exception: Can't unA

我认为如果没有提取函数，就没有有效的方法来实现这一点A。你可以写一个无效的Traversal，但它仍然无法正常工作：

> data A t = A t
> let _A f a = a <$ f (A a)
>
> let a = within (ix 1 . _A) $ zipper ([1,2,3] :: [Int])
> let b = a & _Just . focus .~ A 10
> b ^? _Just . focus
Just (A 10)
> fmap upward b ^? _Just . focus
Just [1,2,3] -- Should be Just [1, 10, 3]