How does topological sorting of kind variables really work?

[Ryan Scott]

Ah, I hadn't considered what effect nondeterminism might have on this.
Here's a quick experiment:

With normal uniques:

    $ /opt/ghc/8.0.1/bin/ghci -XTypeInType -fprint-explicit-kinds
-fprint-explicit-foralls
    GHCi, version 8.0.0.20160411: http://www.haskell.org/ghc/  :? for help
    Loaded GHCi configuration from /home/ryanglscott/.ghci
    λ> data T (a :: j -> k -> l) (b :: (x, y, z)) = MkT
    λ> :info T
    type role T nominal nominal nominal nominal nominal nominal phantom phantom
    data T x y z l k j (a :: j -> k -> l) (b :: (x, y, z)) = MkT
            -- Defined at <interactive>:1:1
    λ> :type MkT
    MkT
      :: forall {x} {y} {z} {l} {k} {j} {a :: j -> k -> l} {b :: (x, y,
                                                                  z)}.
         T x y z l k j a b

With reversed uniques:

    $ /opt/ghc/8.0.1/bin/ghci -XTypeInType -fprint-explicit-kinds
-fprint-explicit-foralls -dunique-increment=-1
-dinitial-unique=16777000
    GHCi, version 8.0.0.20160411: http://www.haskell.org/ghc/  :? for help
    Loaded GHCi configuration from /home/ryanglscott/.ghci
    λ> data T (a :: j -> k -> l) (b :: (x, y, z)) = MkT
    λ> :info T
    type role T nominal nominal nominal nominal nominal nominal phantom phantom
    data T x y z l k j (a :: j -> k -> l) (b :: (x, y, z)) = MkT
            -- Defined at <interactive>:1:1
    λ> :type MkT
    MkT
      :: forall {j} {k} {l} {z} {y} {x} {b :: (x, y, z)} {a :: j
                                                               -> k -> l}.
         T x y z l k j a b

So if :info is to be believed, uniques don't (appear to) have an
effect on the order in which the kind variables are bound in the type
constructor, but they do have an effect with :type (which isn't
surprising, since :type re-generalizes the type signature, and
generalization is precisely what you found to be non-deterministic).

Ryan S.