Finally tagless HTML representation about purescript-halogen

[paf31]

@jdegoes This is the one remaining item on my list for 0.4, and I should have some time tomorrow evening to spend on it.

[jdegoes]

👍

[paf31]

So I made another attempt at this, here:

https://github.com/slamdata/purescript-halogen/tree/final2

I spent two hours on it, but I just kept coming up against typechecker issues:

• I can't use [Attr i] because of impredicative type issues.
• I lose the ability to use Semigroup on Attr i because you can't write things like class (Semigroup (f i)) <= Attr f so I have to use Plus which is a bit weird.
• The type signatures and error messages are baffling. If I were a newcomer, I would probably find it pretty offputting.

I feel like I'm pretty close, but it's not working yet, and at this point, I'm starting to feel like it's a dead end, sadly.

Maybe I can address some of these issues by wrapping things with newtypes, instead of using rank-2 types, so I'll spend a little time on that and see how far I get.

[paf31]

Ok, newtypes fixed pretty much everything here 😄

It's still not entirely done, but things build and the demos work. I'll make a PR so that you can take a look.