I'm doing some work in GHC that requires explicitly applying kinds to the kind variables of a data type. I noticed that the order in which the implicit kind variables are sorted is not as predictable as I originally thought. To illustrate what I mean, here is a GHCi session I had recently (with GHC HEAD): $ /opt/ghc/head/bin/ghci -XTypeInType GHCi, version 8.1.20160412: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/ryanglscott/.ghci > data T (a :: j -> k -> l) (b :: (x, y, z)) = MkT > :t MkT MkT :: forall x y z l k j (a :: j -> k -> l) (b :: (x, y, z)). T a b Much to my surprise, the order of the kind variables wasn't [j, k, l, x, y, z], but rather [x, y, z, l, k, j]! I know that type variables are sorted with a stable topological sort [1], but there must be some other subtlety that I'm missing. How does GHC come up with the order [x, y, z, l, k, j]? Ryan S. ----- [1] http://git.haskell.org/ghc.git/blob/d81cdc227cd487659995ddea577214314c9b4b97...