#14179: "Conflicting family instance" error pretty prints data family instances poorly -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Indeed, the particular pretty printer that's being used in this error message is really broken. If you give GHC these conflicting instances: {{{#!hs module Bug where data family Fam a data instance Fam [a] where Fam1 :: Fam [Int] Fam2 :: Fam [Bool] data instance Fam [a] where Fam3 :: Fam [a] Fam4 :: Fam [Char] }}} Then the error message will print out information that's just plain wrong: {{{ $ ghci Bug.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:6:15: error: Conflicting family instance declarations: Fam [a] = Fam1 | Fam2 -- Defined at Bug.hs:6:15 Fam [a] = Fam3 | Fam4 -- Defined at Bug.hs:9:15 | 6 | data instance Fam [a] where | ^^^ }}} I use the word "particular" above since the general-purpose data family instance pretty printer actually does the right thing: {{{ λ> :i Fam data family Fam a -- Defined at Bug.hs:5:1 data instance Fam [a] where Fam3 :: Fam [a] Fam4 :: Fam [Char] -- Defined at Bug.hs:11:15 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14179#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler