Splitting Ixed/At/Contains into their own package about lens HOT 2 OPEN

From sharing between optics and lens perspective, there isn't much to gain. Ixed in optics and lens look very different.

And that says that something is missing in the design. I don't know what.

For indexed-traversable I do use ifor_ and itraverse, and e.g. ialignWith which builds on top FunctorWithIndex; these classes are not lensy at all. But there's something not right about Ixed without lens or optics.

from lens.

The peculiarity seems to come from the fact that optics is set up to be able to track the kind of optic ix actually returns. We could capture the "zero-or-one" nature with an operation like ix' :: Index m -> m -> Either m (IxValue m, IxValue m -> m). a Traversal written as an infinite sum of get/set pairs (either you get the structure back, or you get one value and a function that accepts one value, or you get two values and a function that accepts two values, or...), then this is like a "zero or one element unfolding" of that infinite sum.

But optics is able to say "ix gives you a lens if you index into a Naperian container", and this encoding doesn't.

from lens.