#15905: Data familes should end in Type -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14645 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I'm not Icelandic, but I can provide one "real-world" use case for this feature from the `singletons` library. It used to be the case that we had about eight or so data types of this shape: {{{#!hs data TyCon1 :: (k1 -> k2) -> (k1 ~> k2) data TyCon2 :: (k1 -> k2 -> k3) -> (k1 ~> k2 ~> k3) data TyCon3 :: (k1 -> k2 -> k3 -> k4) -> (k1 ~> k2 ~> k3 ~> k4) data TyCon4 :: (k1 -> k2 -> k3 -> k4 -> k5) -> (k1 ~> k2 ~> k3 ~> k4 ~> k5) data TyCon5 :: (k1 -> k2 -> k3 -> k4 -> k5 -> k6) -> (k1 ~> k2 ~> k3 ~> k4 ~> k5 ~> k6) data TyCon6 :: (k1 -> k2 -> k3 -> k4 -> k5 -> k6 -> k7) -> (k1 ~> k2 ~> k3 ~> k4 ~> k5 ~> k6 ~> k7) data TyCon7 :: (k1 -> k2 -> k3 -> k4 -> k5 -> k6 -> k7 -> k8) -> (k1 ~> k2 ~> k3 ~> k4 ~> k5 ~> k6 ~> k7 ~> k8) data TyCon8 :: (k1 -> k2 -> k3 -> k4 -> k5 -> k6 -> k7 -> k8 -> k9) -> (k1 ~> k2 ~> k3 ~> k4 ~> k5 ~> k6 ~> k7 ~> k8 ~> k9) }}} We arbitrarily capped off the maximum at eight. With the advent of polymorphic data family return kinds, however, we were able to consolidate all of this down into just three things: {{{#!hs data family TyCon :: (k1 -> k2) -> unmatchable_fun type family ApplyTyCon (f :: k1 -> k2) (x :: k1) :: k3 where ApplyTyCon (f :: k1 -> k2 -> k3) x = TyCon (f x) ApplyTyCon f x = f x type instance Apply (TyCon f) x = ApplyTyCon f x }}} Now we don't need to copy-paste `TyCon` multiple times, and better yet, we don't have to arbitrarily set a maximum of eight, since `ApplyTyCon` lets us go to whatever arity we desire. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15905#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler