#15870: No skolem info panic -------------------------------------+------------------------------------- Reporter: sheaf | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.6.3 Component: Compiler (Type | Version: 8.6.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * priority: normal => highest * milestone: => 8.6.3 Comment: First, this appears to be a regression from GHC 8.4, which simply gives an error message: {{{ $ /opt/ghc/8.4.4/bin/ghci Bug.hs -XTypeInType GHCi, version 8.4.4: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:30:34: error: • Expected kind ‘Optic a’, but ‘g2’ has kind ‘Optic b’ • In the second argument of ‘Get’, namely ‘g2’ In the type ‘Get a g2’ In the type instance declaration for ‘Get’ | 30 | type Get a (g1 :.: g2) = Get a g2 | ^^ }}} So that's not good. I'll mark this as highest priority as a result. Second, the error message: {{{ • Expected kind ‘Optic b’, but ‘g2’ has kind ‘Optic (Get a g1)’ • In the second argument of ‘Gettable’, namely ‘g2’ In the instance declaration for ‘Gettable a (g1 :.: g2)’ | 20 | , Gettable b g2 | }}} Is actually expected behavior (at least, at the moment). Even though you've written `b ~ Get a g1`, GHC isn't able to make use of this fact at the type level yet. See #12677/#15710 for the tickets tracking this infelicity. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15870#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler