#10598: DeriveAnyClass and GND don't work well together -------------------------------------+------------------------------------- Reporter: osa1 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Description changed by osa1: Old description:
I think we definitely have a bug here, but I'm not sure what it really is.
Here's the program:
{{{ newtype MyMaybe a = MyMaybe (Maybe a) deriving (Functor, Show)
main = print $ MyMaybe $ Just (10 :: Int) }}}
I'm using GHC 7.10.1.
{{{ ➜ deriveany_bug ghc --make -fforce-recomp Test.hs -XDeriveAnyClass -XGeneralizedNewtypeDeriving [1 of 1] Compiling Main ( Test.hs, Test.o )
Test.hs:2:13: Can't make a derived instance of ‘Functor MyMaybe’ (even with cunning newtype deriving): You need DeriveFunctor to derive an instance for this class Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension In the newtype declaration for ‘MyMaybe’ }}}
Just to try, changing argument order:
{{{ ➜ deriveany_bug ghc --make -fforce-recomp Test.hs -XGeneralizedNewtypeDeriving -XDeriveAnyClass [1 of 1] Compiling Main ( Test.hs, Test.o )
Test.hs:2:13: Can't make a derived instance of ‘Functor MyMaybe’ (even with cunning newtype deriving): You need DeriveFunctor to derive an instance for this class Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension In the newtype declaration for ‘MyMaybe’ }}}
It works fine if I remove `DeriveAnyClass`:
{{{ ➜ deriveany_bug ghc --make -fforce-recomp Test.hs -XGeneralizedNewtypeDeriving [1 of 1] Compiling Main ( Test.hs, Test.o ) Linking Test ... }}}
GHC HEAD is failing in exactly the same way.
User manual is saying this in 7.5.6:
In case you try to derive some class on a newtype, and -XGeneralizedNewtypeDeriving is also on, -XDeriveAnyClass takes precedence.
But then why is it telling me to enable `GeneralizedNewtypeDeriving` in the error message? Even if I already enabled it?
Also, maybe it could try `GND` when `DeriveAnyClass` fails? Because the doc is saying `DeriveAnyClass` has precedence but doesn't specify what happens if it fails.
In case you try to derive some class on a newtype, and -XGeneralizedNewtypeDeriving is also on, -XDeriveAnyClass takes
New description: I think we definitely have a bug here, but I'm not sure what it really is. Here's the program: {{{ newtype MyMaybe a = MyMaybe (Maybe a) deriving (Functor, Show) main = print $ MyMaybe $ Just (10 :: Int) }}} I'm using GHC 7.10.1. {{{ ➜ deriveany_bug ghc --make -fforce-recomp Test.hs -XDeriveAnyClass -XGeneralizedNewtypeDeriving [1 of 1] Compiling Main ( Test.hs, Test.o ) Test.hs:2:13: Can't make a derived instance of ‘Functor MyMaybe’ (even with cunning newtype deriving): You need DeriveFunctor to derive an instance for this class Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension In the newtype declaration for ‘MyMaybe’ }}} Just to try, changing argument order: {{{ ➜ deriveany_bug ghc --make -fforce-recomp Test.hs -XGeneralizedNewtypeDeriving -XDeriveAnyClass [1 of 1] Compiling Main ( Test.hs, Test.o ) Test.hs:2:13: Can't make a derived instance of ‘Functor MyMaybe’ (even with cunning newtype deriving): You need DeriveFunctor to derive an instance for this class Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension In the newtype declaration for ‘MyMaybe’ }}} It works fine if I remove `DeriveAnyClass`: {{{ ➜ deriveany_bug ghc --make -fforce-recomp Test.hs -XGeneralizedNewtypeDeriving [1 of 1] Compiling Main ( Test.hs, Test.o ) Linking Test ... }}} GHC HEAD is failing in exactly the same way. User manual is saying this in 7.5.6: precedence. But then why is it telling me to enable `GeneralizedNewtypeDeriving` in the error message? Even if I already enabled it? Also, maybe it could try `GND` when `DeriveAnyClass` fails? Because the doc is saying `DeriveAnyClass` has precedence but doesn't specify what happens if it fails. EDIT: I'd like to work on this myself if experts here help me figuring the right behavior here. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler