#11594: closed empty type families fully applied get reduced lazily when in a constraint tuple and fully applied -------------------------------------+------------------------------------- Reporter: carter | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by carter): I guess it could be viewed as a feature request. I need to play around with the 8.0 / master a bit to have a clear sense of if TypeError alone suffices, but I DO think that from a user / usability perspective, the fact that we DO present the definition of TypeError as being an empty closed type family in GHC.TypeLits will be confusing for some users. I think either a) the documentation in http://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/libraries/base-4.9.0.0 /GHC-TypeLits.html#g:4 (the GHC.TypeLits module) should be changed to clearly articulate that TypeError has special wired-in behavior that can't be replicated in userland, (including what special treatment it gets versus userland type families that are written simularly) OR b) anything that has a reasonable looking source definition in base that isn't the infinite loop style found in http://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/libraries/ghc- prim-0.5.0.0/GHC-Prim.html and not explicitly documented as being a deeply magical / wired-in behavior really should be replicable in userland zooming out: its probably the case that for me, TypeError will probably be fine for me, but my main concern is about having a clearer understanding of whats a magical special case and what isnt', and how the semantics of the pieces of magics is clearly articulated -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11594#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler