[Haskell-beginners] why is there no typeclass common to all containers?

[Simon Jakobi]

There are several packages that define typeclasses for containers and,
in particular, "map-like" things:

* keys (no construction, no insertion)
* Data.Containers from the mono-traversable package
* EdisonAPI
* https://github.com/commercialhaskell/jump (still in the works)

Another somewhat related package is focus, which defines a common
abstraction for various operations on a key-value pair in a map.