Diffing and then patching a Set fails

[thomasjm]

I'm seeing a problem when I try to diff a change to a Set Text. I think the issue is that the diff is performed by turning the set into JSON and passing it to Data.Aeson.Diff, which then treats the set like a list, and produces a patch with an index that's too large for the FieldLens instance for Set.

Note that this is case-sensitive; if you switch the roles of "a" and "b" then it succeeds.

I think maybe a valid fix would be for the FieldLens Set instance to accept a key equal to the size of the set?

{-# LANGUAGE DeriveGeneric, TemplateHaskell, OverloadedStrings, QuasiQuotes #-}

import Data.Aeson
import Data.Aeson.Diff
import Data.Aeson.Diff.Generic as ADG
import Data.Set as Set
import Data.String.Interpolate
import Relude

main :: IO ()
main = do
  let orig = Set.fromList ["a" :: Text]
  let new = Set.fromList ["a", "b" :: Text]
  let p = diff (toJSON orig) (toJSON new)
  putStrLn [i|Patch: #{p}|]

  let result = ADG.patch p orig
  putStrLn [i|Result: #{result}|]

Output:

> main
Patch: Patch {patchOperations = [Add {changePointer = Pointer {pointerPath = [AKey 1]}, changeValue = String "b"}]}
Result: Error "Operation Add on /1 failed: Index out of bounds"