migration-guide-0.4.md

Migration guide to optics-0.4

FunctorWithIndex instances

In optics-0.4 the FunctorWithIndex, FoldableWithIndex and TraversableWithIndex type classes have been migrated to a new package, indexed-traversable.

Beware: the lens package (versions <5) 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 optics-0.3 users, you may write

-- from indexed-traversable
import Data.Functor.WithIndex

-- from optics-core
import qualified Optics.Core as O

-- 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 = ...

-- optics-core <0.4 instance, note the !
#if !MIN_VERSION_optics_core(0,4,0)
instance O.FunctorWithIndex     Int MySeq where imap = imap
instance O.FoldableWithIndex    Int MySeq where ifoldMap = ifoldMap
instance O.TraversableWithIndex Int MySeq where itraverse = itraverse
#endif

In other words, always provide indexed-traversable instances. If your package depends on optics(-core) and allows optics-0.3, you should additionally provide instances for optics-0.3 type classes that can reuse the indexed-traversable instances.