Support for explicit downcasting indexed optics to regular ones

[arybczak]

Proof of concept for #25 for now (in particular I think we should have a class with associated function unIx instead of unIxTraversal etc.

I think it can look better when representing indices as in #41, then regular versions have to accept optics with any list of indices instead of two free type parameters.

@phadej thoughts?

[phadej]

Functpr

Is renaming to avoid clashes with lens?

[arybczak]

Not really, I just figured that since we have IxProfunctor and indexed optics start with Ix, classes can be the same for consistency.

[phadej]

I feel uneasy about this.

Do we still have

traverseOf traversed = traverse
traverseOf (traversed % traversed) = traverse . traverse

(I think inspection-testing)

But if this works, then #41 can be tweaked even further into nicer form indeed.

[arybczak]

Yeah, nothing changes with regard to operations on regular optics, the extra constraint is just so that indexed variants can be used as regular ones (as Ix* instances for Forget, Star and FunArrow simply ignore indices).

[arybczak]

Ah, you mean code generation? I didn't check.

[phadej]

@arybczak could you pull the "Ranem indexed classes" (fixing the typo) into own PR so we could merge it right away (If that's something we want to do, I have no strong opinion about)

[arybczak]

Right, I'll do that later today.

Yes, we want explicit (not implicit though as in lens) downcasting.

[phadej]

@arybczak I'll push inspection tests very soon.

[phadej]

@arybczak I just merged the inspection tests I mentioned, if they still pass with your patch, I think this is a good way forward.

[arybczak]

Looks like they pass. I force pushed the UnindexableOptic class and removed the commit that renames the classes.