Haskell语言学习笔记（44）Lens（2）

preview, review

Prelude Control.Lens> view _Left (Left "abc")
"abc"
Prelude Control.Lens> view _Right (Right "abc")
"abc"
Prelude Control.Lens> view _Just (Just "abc")
"abc"
Prelude Control.Lens> preview _Left (Left "abc")
Just "abc"
Prelude Control.Lens> review _Left "abc"
Left "abc"

• preview 和 review 函数处理 Either 这样的和类型
  preview 函数向上走一个分支。
  review 函数向下走一个分支。

preview, review 的操作符版本

Prelude Control.Lens> Left "abc" ^. _Left
"abc"
Prelude Control.Lens> Left "abc" ^? _Left
Just "abc"
Prelude Control.Lens> Right "abc" ^? _Left
Nothing
Prelude Control.Lens> Right "abc" ^? _Right
Just "abc"
Prelude Control.Lens> _Left # "abc"
Left "abc"

preview l x ≡ x ^? l
review l x ≡ l # x

toListOf 及其操作符版本

Prelude Control.Lens> toListOf traverse [1,2,3]
[1,2,3]
Prelude Control.Lens> toListOf (traverse.traverse) [[1,2],[3]]
[1,2,3]
Prelude Control.Lens> toListOf both (1,2)
[1,2]
Prelude Control.Lens> toListOf _1 (4, 1)
[4]
Prelude Control.Lens> [[1,2],[3]] ^..traverse.traverse
[1,2,3]
Prelude Control.Lens> (1,2) ^..both
[1,2]
Prelude Control.Lens> (4, 1) ^.._2
[1]