| prefer, such as only exporting the Coerce instance if all the | constructors are exported, it seems that the ship sailed on these Coercible is relatively recent; I don't think we should regard it as cast in stone. But yes, the Coerbible instance of a newtype is only available when the data constructor for the newtype is lexically in scope. Simon | -----Original Message----- | From: davidterei@gmail.com [mailto:davidterei@gmail.com] On Behalf Of | David Terei | Sent: 10 April 2015 09:38 | To: Simon Peyton Jones | Cc: Omari Norman; haskell Cafe; ghc-devs@haskell.org | Subject: Re: Generalized Newtype Deriving not allowed in Safe Haskell | | I'll prepare a patch for the userguide soon. | | As for something better, yes I think we can and should. It's on my todo | list :) Basically, the new-GND design has all the mechanisms to be | safe, but sadly the defaults are rather worrying. Without explicit | annotations from the user, module abstractions are broken. This is why | we left GND out of Safe Haskell for the moment as it is a subtle and | easy mistake to make. | | If the module contained explicit role annotations then it could be | allowed. The discussion in | https://ghc.haskell.org/trac/ghc/ticket/8827 has other solutions that I | prefer, such as only exporting the Coerce instance if all the | constructors are exported, it seems that the ship sailed on these | bigger changes sadly. | | Cheers, | David | | On 9 April 2015 at 00:56, Simon Peyton Jones <simonpj@microsoft.com> | wrote: | > There is a long discussion on | > https://ghc.haskell.org/trac/ghc/ticket/8827 | > about whether the new Coercible story makes GND ok for Safe Haskell. | > At a type-soundness level, definitely yes. But there are other | > less-clear-cut issues like “breaking abstractions” to consider. The | > decision on the ticket | > (comment:36) seems to be: GND stays out of Safe Haskell for now, but | > there is room for a better proposal. | > | > | > | > I don’t have an opinion myself. David Terei and David Mazieres are in | > the driving seat, but I’m sure they’ll be responsive to user input. | > | > | > | > However, I think the user manual may not have kept up with #8827. | The | > sentence “GeneralizedNewtypeDeriving — It can be used to violate | > constructor access control, by allowing untrusted code to manipulate | > protected data types in ways the data type author did not intend, | > breaking invariants they have established.” vanished from the 7.8 | > user manual (links below). Maybe it should be restored. | > | > | > | > Safe Haskell aficionados, would you like to offer a patch for the | manual? | > And maybe also a less drastic remedy than omitting GND altogether? | > | > | > | > Simon | > | > | > | > From: Omari Norman [mailto:omari@smileystation.com] | > Sent: 09 April 2015 02:44 | > To: haskell Cafe | > Subject: Generalized Newtype Deriving not allowed in Safe Haskell | > | > | > | > When compiling code with Generalized Newtype Deriving and the | > -fwarn-unsafe flag, I get | > | > | > | > -XGeneralizedNewtypeDeriving is not allowed in Safe Haskell | > | > | > | > This happens both in GHC 7.8 and GHC 7.10. | > | > | > | > I thought I remembered reading somewhere that GNTD is now part of the | > safe language? The GHC manual used to state that GNTD is not allowed | > in Safe | > Haskell: | > | > | > | > https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/safe- | ha | > skell.html#safe-language | > | > | > | > But this language on GNTD not being part of the safe language was | > removed in the 7.8 manual: | > | > | > | > https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/safe- | ha | > skell.html#safe-language | > | > | > | > The GHC release notes don't say anything about this one way or the | other. | > Thoughts? | > | > | > _______________________________________________ | > ghc-devs mailing list | > ghc-devs@haskell.org | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | >