The `imapped`, `ifolded` and `itraversed` methods are now top-level functions.
If you are not defining these methods in your instances,
you don't need to change your definitions.
Beware: the `optics-core` package (versions <0.4) defines similar classes,
and will also migrate to use `indexed-traversable` classes. Therefore, you
might get duplicate instance errors if your package defines both.
If you define your own `FunctorWithIndex` etc. instances,
we recommend that you depend directly on the `indexed-traversable` package.
If you want to continue support `lens-4` users, you may write
```haskell
-- from indexed-traversable
import Data.Functor.WithIndex
-- from lens
import qualified Control.Lens as L
-- your (indexed) container
data MySeq a = ...
-- indexed-traversable instance
instance FunctorWithIndex Int MySeq where imap = ...
instance FoldableWithIndex Int MySeq where ifoldMap = ...
instance TraversableWithIndex Int MySeq where itraverse = ...
-- lens <5 instance, note the !
#if !MIN_VERSION_lens(5,0,0)
instance L.FunctorWithIndex Int MySeq where imap = imap
instance L.FoldableWithIndex Int MySeq where ifoldMap = ifoldMap
instance L.TraversableWithIndex Int MySeq where itraverse = itraverse
#endif
```
In other words, always provide `indexed-traversable` instances.
If your package depends on `lens` and allows `lens-4`,
you should additionally provide instances for `lens-4` type classes
that can reuse the `indexed-traversable` instances.
* Make the functions in `Control.Lens.TH` work more robustly with poly-kinded
data types. This can cause a breaking change under certain situations:
* TH-generated optics for poly-kinded data types are now much more likely to
mention kind variables in their definitions, which will require enabling
the `PolyKinds` extension at use sites in order to typecheck.
* Because TH-generated optics now quantify more kind variables than they did
previously, this can affect the order of visible type applications.
* Generalize the types of `generic` and `generic1` to allow type-changing
updates. If you wish to use the old, more restricted types of these
functions, use `simple . generic` or `simple . generic1` instead.
* Add `Control.Lens.Profunctor` with conversion functions to and from
profunctor optic representation.
* Add `Control.Lens.Review.reviewing`, which is like `review` but with a more
polymorphic type.
* Mark `Control.Lens.Equality` as Trustworthy.
* The build-type has been changed from `Custom` to `Simple`.
To achieve this, the `doctests` test suite has been removed in favor of using [`cabal-docspec`](https://github.com/phadej/cabal-extras/tree/master/cabal-docspec) to run the doctests.
* Use `alterF` in `At (HashMap k)` instance implementation.
* Use `alterF` in `At` and `Contains` instances for `Set`, `IntSet`, and
`HashSet`.
* Avoid re-inserting keys already present in `ix` for `Set`, `IntSet`,
and `HashSet`. For `Set` and `HashSet`, this changes the semantics
slightly; if the user-supplied key is `==` to one already present in
the set, then the latter will not be replaced in the result.
* Consume `()` values lazily in `Control.Lens.At`.
