Question

我在与lens和zippers进行斗争。请考虑以下ghci

中运行的代码

> 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)

拥有data A t = A t，如何创建拉链类型，如：Control.Monad.MonadPlus m => m (Zipper Top Int [Int] :>> A Int)？

我尝试了within (ix 1 . to A) $ zipper ([1,2,3] :: [Int])，但却出错了：

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Iso并与之合作。在ghci：

> 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(\(A a) -> a)的原因是您可以退出。

> 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

我认为没有提取A的功能，这是一种有效的方法。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]

用`到`镜头踩到拉链

1 个答案: