Dear Devs, I've tried to ask this in the ($) thread, but it was totally offtopic there and I was ignored just as I deserved :-) Consider the following example. wojtek@Desktop2016:~/src/he$ cat kinds.hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} data K = A | B f :: (A :: K) -> (B :: K) f _ = undefined wojtek@Desktop2016:~/src/he$ /opt/ghc/head/bin/ghc kinds.hs [1 of 1] Compiling Main ( kinds.hs, kinds.o ) kinds.hs:6:6: error: • Expected a type, but ‘'A’ has kind ‘K’ • In the type signature: f :: (A :: K) -> (B :: K) kinds.hs:6:18: error: • Expected a type, but ‘'B’ has kind ‘K’ • In the type signature: f :: (A :: K) -> (B :: K) As Roman kindly (!) explained to me some time ago, GHC really means "Expected a type of kind '*' (or '#')..." Now that GHC is apparently undergoing a major overhaul of its internals, would it be possible to allow types of various kinds in functions? Would it make sense? May I file a ticket? -- Kind regards, Wojtek Narczynski