@rae: A not-so-quick look at Quick Look impredicativity

I introduce the new ImpredicativeTypes extension and show a way to use it to make indexing into binary trees convenient.

[monoidal_]

The issue with defaulting to Any at 5:14 is GHC #19110.

[malkdk]

8:25 that *is* a really nice-looking arbitrary tree of int’s 😄

[virkony]

When I first saw inability to use forall without proper constructor I though there were some idea behind it. E.g. you have to explicitly pack polymorphic type behind something that have interface that allows to manage it without knowing actual type (e.g. using pointer to class with virtual dtor in C++). And you can't use newtype exactly because there is some overhead associated with it.

[welltypedwitch]

It's great to see a use case for ImpredicativeTypes that is more realistic than "putting ids or lenses in lists"! I'm still left with two questions though:

1) Why can't GHC check list literals but can check explicit uses of (:) and []? Was that just an oversight since the extension is still pretty new?

2) Couldn't you do the same thing without ImpredicativeTypes by defining TreeIndex as a newtype?

[mck]

In TreeIndex the 'a' looks like it's an existential variable, but I don't think you use that word here. Will this extension allow using existentials where it was not possible before?