Make widgets as powerful as components about purescript-halogen

[jdegoes]

In fact, I'd argue that at some level, a widget must be able to be wrapped up into a component, so that it can accept and generate messages exactly like a component. Strictly one-way communication will not work for many use cases and will necessitate messy workarounds.

[paf31]

I think it should be possible to wrap widgets as components using some kind of helper function.

The tricky part right now is that I'm running into variance issues trying to make Widget into a Functor. The type of the renderer becomes:

HTML p i ~ (i -> Eff eff Unit) -> (p -> Widget eff i) -> VTree

which is invariant in i. Widget essentially has become

type Widget eff i = (i -> Eff eff Unit) -> VTree

which is covariant.

Maybe I've just been staring at this for too long, but I feel like these pieces should be fitting together.

_Edit: inlining p gives

HTML (Widget eff i) i ~ (i -> Eff eff Unit) -> VTree

which is covariant, but this is tricky because of the finally tagless approach.

[jdegoes]

which is covariant, but this is tricky because of the finally tagless approach.

Well, finally tagless is subservient to good composable abstractions (IMO), so if it proves an obstacle, we can always go back to initial.

[jdegoes]

This ticket should really be retitled, "Widgets should be as powerful as Components".

[paf31]

Fixed in #58. The Ace editor example is a good demo of this.