#11581: TypeError requires UndecidableInstances unnecessarily -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.0.1-rc2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * cc: diatchki (added) * failure: None/Unknown => GHC rejects valid program @@ -2,1 +2,1 @@ - {{{ + {{{#!hs @@ -21,1 +21,1 @@ - {{{ + {{{#!hs New description: The example from the `TypeError` documentation {{{#!hs type family ByteSize x where ByteSize Word16 = 2 ByteSize Word8 = 1 ByteSize a = TypeError (Text "The type " :<>: ShowType a :<>: Text " is not exportable.") }}} requires `UndecidableInstances`: {{{ BS.hs:11:5: error: • The type family application ‘(TypeError ...)’ is no smaller than the instance head (Use UndecidableInstances to permit this) • In the equations for closed type family ‘ByteSize’ In the type family declaration for ‘ByteSize’ }}} Obviously there's no real danger of undecidability here. I tried changing the declaration of `TypeError` to {{{#!hs type family TypeError :: ErrorMessage -> b where }}} but it didn't help. Same error. Is that a bug? -- Comment: Iavor, perhaps you can comment here? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11581#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler