#14059: COMPLETE sets don't work at all with data family instances -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: mpickering (added) Comment: Here's one architectural issue: `COMPLETE` sets currently aren't designed with data family instances in mind. Why? Because you can optionally give the name of a tycon for a `COMPLETE` set, e.g., {{{#!hs {-# COMPLETE False, TooGoodToBeTrue :: Boolean #-} }}} However, this necessarily needs to be more involved for data family instances, because we're dealing with a proper type, not just a tycon. Put differently, it wouldn't be enough to say: {{{#!hs {-# COMPLETE SFalse, STrue :: Sing #-} }}} This is wrong, since we don't want a `COMPLETE` set for `Sing (a :: k)`, we want a `COMPLETE` set for `Sing (a :: Bool)`. But GHC won't let us use: {{{#!hs {-# COMPLETE SFalse, STrue :: Sing (z :: Bool) #-} }}} {{{ Bug.hs:20:47: error: parse error on input ‘(’ | 20 | {-# COMPLETE SFalse, STooGoodToBeTrue :: Sing (z :: Bool) #-} | }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14059#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler