#10835: Regression in standalone Data deriving for phantom types -------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Description changed by Feuerbach: Old description:

Thise code

{{{#!hs {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving, PolyKinds #-} module M where import Data.Data import Data.Typeable

data Phantom t = Phantom deriving Typeable deriving instance Typeable t => Data (Phantom t)

}}}

compiles with 7.8.4 but not with 7.10.2. The error is

{{{ data.hs:8:1: Could not deduce (Typeable Phantom) arising from the superclasses of an instance declaration from the context (Typeable t) bound by the instance declaration at data.hs:8:1-48 In the instance declaration for ‘Data (Phantom t)’ }}}

PolyKinds here is a hack I came up with to make this compile with 7.8; but ideally, this code should compile whether PolyKinds is enabled or not.

New description: Thise code {{{#!hs {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving, PolyKinds #-} module M where import Data.Data import Data.Typeable data Phantom t = Phantom deriving Typeable deriving instance Typeable t => Data (Phantom t) }}} compiles with 7.8.4 but not with 7.10.2. The error is {{{ data.hs:8:1: Could not deduce (Typeable Phantom) arising from the superclasses of an instance declaration from the context (Typeable t) bound by the instance declaration at data.hs:8:1-48 In the instance declaration for ‘Data (Phantom t)’ }}} PolyKinds here is a hack I came up with to make this compile with 7.8 (without it, the code doesn't compile with either version); but ideally, this code should compile whether PolyKinds is enabled or not. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10835#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10835: Regression in standalone Data deriving for phantom types -------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: bgamari Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: 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 bgamari): * status: new => closed * resolution: => fixed * milestone: => 8.0.1 Comment: With GHC 8.0 the polykinded example can be made to compile by adding a `Typeable` constraint on the kind of `t`, {{{#!hs {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving, PolyKinds, TypeInType #-} module M where import Data.Data import Data.Typeable data Phantom t = Phantom deriving Typeable deriving instance (Typeable k, Typeable (t::k)) => Data (Phantom t) }}} It also compiles without `PolyKinds`, although only with a `Data t` constraint, {{{#!hs {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-} module M where import Data.Data import Data.Typeable data Phantom t = Phantom deriving Typeable deriving instance (Data t) => Data (Phantom t) }}} If one replaces the `Data` constraint with `Typeable` typechecking fails with, {{{ Hi2.hs:8:1: error: • Could not deduce (Data t) arising from a use of ‘f’ from the context: Typeable t bound by the instance declaration at Hi2.hs:8:1-50 or from: Typeable t1 bound by the type signature for: dataCast1 :: Typeable t1 => (forall d. Data d => c (t1 d)) -> Maybe (c (Phantom t)) at Hi2.hs:8:1-50 Possible fix: add (Data t) to the context of the type signature for: dataCast1 :: Typeable t1 => (forall d. Data d => c (t1 d)) -> Maybe (c (Phantom t)) or the instance declaration • In the first argument of ‘gcast1’, namely ‘f’ In the expression: gcast1 f In an equation for ‘dataCast1’: dataCast1 f = gcast1 f When typechecking the code for ‘dataCast1’ in a derived instance for ‘Data (Phantom t)’: To see the code I am typechecking, use -ddump-deriv }}} The reason for this is that GHC generates the following instance, {{{#!hs instance Typeable t => Data (Phantom t) where ... dataCast1 :: forall s c. (Typeable s) => (forall d. Data d => c (s d)) -> Maybe (c (Phantom t)) dataCast1 f = Data.Typeable.gcast1 f -- where Data.Typeable.gcast1 :: forall k k1 (c :: k -> *) (t :: k1 -> k) (t' :: k1 -> k) (a :: k1). (Typeable t, Typeable t') => c (t a) -> Maybe (c (t' a)) }}} Consequently the derived instance forces the `t ~ d` (WRT the type binders of `dataCast1` above). `dataCast1` requires `Data d`, hence the error. This all seems to be working as expected in GHC 8.0.1. Indeed, 7.8.4 also appears to require a `Data t` constraint despite the claim in the ticket summary, so I think all is well here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10835#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10835: Regression in standalone Data deriving for phantom types -------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: bgamari Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: deriving 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): * keywords: => deriving * cc: deriving (removed) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10835#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler