What is the approach to implementing mapMaybe with lenses?

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The approach to implementing mapMaybe with lenses involves using the Traversal' function from the lens library. Here are the steps:

1. Define a Traversal' over the elements in the structure that you want to apply the mapMaybe function to. For example, if you want to apply mapMaybe to a list, you can define a traversal over each element in the list using the traversed function.

2. Define a function that takes an element from the traversal and returns a Maybe value. This function will be passed to the mapMaybeOf function.

3. Use the mapMaybeOf function to apply the function defined in step 2 to each element in the traversal, discarding any Nothing values.

4. Use the toListOf function to extract the modified elements from the traversal as a list.

Here is an example implementation of the mapMaybe function using lenses:

import Control.Lens

mapMaybeOf :: Traversal' s a -> (a -> Maybe b) -> s -> [b]
mapMaybeOf l f s = toListOf (l . traverse . filtered isJust . to fromJust . to f) s

-- Usage:
-- > mapMaybeOf traversed (\x -> if x `mod` 2 == 0 then Just x else Nothing) [1,2,3,4]
-- [2,4]
-- > mapMaybeOf (traversed . _2) (\x -> if x `mod` 2 == 0 then Just x else Nothing) [("a",1),("b",2),("c",3),("d",4)]
-- [2,4]

In this implementation, traverse is used to traverse the elements in the structure, filtered is used to exclude the Nothing values, fromJust is used to extract the wrapped Just value, and to is used to apply the function f to each element. The final list of modified elements is extracted using toListOf.