GHCi's :kind doesn't work with wildcards
Witness this GHCi session:
rae:20:35:37 ~/ghc/ghc-head/ghc> ~/ghc/ghc-head/inplace/bin/ghc-stage2 --interactive GHCi, version 8.7.20180716: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /Users/rae/.ghc/ghci.conf Prelude> :set -XPartialTypeSignatures Prelude> import Data.Proxy Prelude Data.Proxy> :k Proxy _ Proxy _ :: * Prelude Data.Proxy> :k Proxy (Maybe :: _) <interactive>:1:8: error: • Expecting one more argument to ‘Maybe’ Expected kind ‘_’, but ‘Maybe’ has kind ‘* -> *’ • In the first argument of ‘Proxy’, namely ‘(Maybe :: _)’ In the type ‘Proxy (Maybe :: _)’
It seems we're not doing the correct wildcard thing here: GHCi should report that
_ should be
Type -> Type.
I believe that this is because
tcWildCardBinders is called from
TcRnDriver.tcRnType, which is the implementation of GHCi's
:kind. This is GHC's only call of
tcWildCardBinders. All other places use
tcWildCardBindersX newWildTyVar, which makes vastly more sense.
I would just fix this myself, but it smells intentional. Does anyone know why we have all this?
(Credit to @int-index who found this bug.)