#15872: Odd pretty printing of equality constraint in kind ('GHC.Types.Eq# <>) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #13933 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #13933 Comment: See #13933 for another way to trigger the pretty-printing oddities uncovered here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15872#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15872: Odd pretty printing of equality constraint in kind ('GHC.Types.Eq# <>) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #13933 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): In GHC HEAD (as of commit 2257a86daa72db382eb927df12a718669d5491f8), you get the following error when compiling the original program: {{{ GHCi, version 8.7.20181129: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( ../Bug.hs, interpreted ) ../Bug.hs:10:1: error: • Illegal constraint in a kind: forall (a :: WHICH). (a ~ 'OP) => * -> * -> * • In the data type declaration for ‘Fun’ | 10 | data Fun :: forall (a :: WHICH). a ~ OP => Type -> Type -> Type where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... }}} Which would suggest that this issue is now moot. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15872#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15872: Odd pretty printing of equality constraint in kind ('GHC.Types.Eq# <>) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #12102, #13933 | Differential Rev(s): Phab:D5397 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott ): In [changeset:"73cce63f33ee80f5095085141df9313ac70d1cfa/ghc" 73cce63f/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="73cce63f33ee80f5095085141df9313ac70d1cfa" Fix #12102/#15872 by removing outdated users' guide prose Summary: In the beginning, #12102 (and #15872, which is of a similar ilk) were caused by a poor, confused user trying to use code that looks like this (with a constraint in the kind of a data type): ```lang=haskell type family IsTypeLit a where IsTypeLit Nat = 'True IsTypeLit Symbol = 'True IsTypeLit a = 'False data T :: forall a. (IsTypeLit a ~ 'True) => a -> * where MkNat :: T 42 MkSymbol :: T "Don't panic!" ``` Many bizarre GHC quirks (documented in those tickets) arose from this sort of construction. Ultimately, the use of constraints in data type kinds like this has made a lot of people very confused and been widely regarded as a bad move. Commit 2257a86daa72db382eb927df12a718669d5491f8 finally put this feature out of its misery, so now the code above simply errors with `Illegal constraint in a kind`. As a result, the aforementioned tickets are moot, so this patch wraps a bow on the whole thing by: 1. Removing the (now outdated) section on constraints in data type kinds from the users' guide, and 2. Adding a test case to test this code path. Test Plan: make test TEST=T12102 Reviewers: goldfire, simonpj, bgamari, tdammers Reviewed By: tdammers Subscribers: tdammers, rwbarton, carter GHC Trac Issues: #12102, #15872 Differential Revision: https://phabricator.haskell.org/D5397 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15872#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15872: Odd pretty printing of equality constraint in kind ('GHC.Types.Eq# <>) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Poor/confusing | Test Case: error message | typecheck/should_fail/T12102 Blocked By: | Blocking: Related Tickets: #12102, #13933 | Differential Rev(s): Phab:D5397 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: closed => new * resolution: fixed => * milestone: 8.8.1 => Comment: Commit [https://gitlab.haskell.org/ghc/ghc/commit/6cce36f83aec33d33545e0ef2135894d22... 6cce36f83aec33d33545e0ef2135894d22dff5ca] (`Add AnonArgFlag to FunTy`) added back the ability to have equality constraints in kinds. Unfortunately, the issues in the original description persist: {{{ GHCi, version 8.9.20190224: https://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Ok, one module loaded. λ> :i MkFun type role Fun nominal representational representational data Fun (a1 :: a ~ 'OP) b c where MkFun :: b -> c -> Fun 'GHC.Types.Eq# <> b c -- Defined at Bug.hs:11:3 λ> :k Fun Fun :: (a ~ 'OP) -> * -> * -> * }}} In fact, the situation is arguably //worse// now, since `:k Fun` reports the entirely bogus kind `Fun :: (a ~ 'OP) -> * -> * -> *`. (See also #12102.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15872#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler