Support 'thunk' from virtual-dom about purescript-halogen

[jdegoes]

Our use cases include purescript-ace, purescript-echarts, and probably others. Having a modular, clean, and robust way of using these pre-built DOM-native components is quite important.

[paf31]

The simplest approach seems to be something like adding a new constructor to HTML which prevents redraws using thunk:

data HTML i
  = NoRedraw (Unit -> HTML i)
  | Text ...

We could then extend that in some simple way, like adding a String identifier which could be stuck in a data- attribute and used to implement limited updates:

type Hash = String

data HTML i
  = Hashed String (Unit -> HTML i)
  | Text ...

If the hashes are equal, we assume the VTrees are equal and use the previous VTree. Otherwise, we rerender the HTML structure.

The issue is that thunk only gives us access to the previous VTree, not the HTML structure it came from.

[paf31]

Also, @jdegoes, I'm not a vdom expert yet, but I feel like the "widget" concept might be useful for the examples you mentioned. I'll write up a separate issue for that.

https://github.com/Matt-Esch/virtual-dom/blob/master/docs/widget.md

[garyb]

I'd agree with that, thunk is for custom patching logic, widget is more for things like Ace.