Folding or concatMap-ing an Aeson Array via lenses

问题

I've been starting at https://www.stackage.org/haddock/lts-12.1/lens-aeson-1.0.2/Data-Aeson-Lens.html and https://www.stackage.org/haddock/lts-12.1/lens-4.16.1/Control-Lens-Fold.html trying to figure out how to write an expression that allows me to construct something of the following type:

import Data.Aeson as A

functionIWant 
  :: (Vector A.Value)
  -> (A.Value -> [a])
  -> [a]

回答1:

There are two lens functions that are used to "lift" regular Foldable-related functions to the lensy world:

• folded that folds over all the elements of a Foldable container, in this case it would be the Vector.
• folding that lifts a function that returns a Foldable result.

You can compose them into a new Fold and then use toListOf to obtain the final list.

There is also an analogue of folded for Traversable, called traversed.

来源：https://stackoverflow.com/questions/51906616/folding-or-concatmap-ing-an-aeson-array-via-lenses