#14045: Data family instances must list all patterns of family, despite documentation's claims to the contrary -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.3 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: indexed- valid program | types/should_compile/T14045, | indexed-types/should_fail/T14045a Blocked By: | Blocking: Related Tickets: #12369 | Differential Rev(s): Phab:D3804 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Ah I missed the reference to `Note [Arity of data families]`. Thanks. But {{{ This instance is very nearly equivalent to data instance Sing (a :: Bool) where ... but without the last pattern, we have an under-saturated data family instance. On its own, this example is not compelling }}} Can you say more precisely what /is/ the difference? Is it only the newtype-eta thing? Or is there anything else? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14045#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler