Generalized Newtype Deriving not allowed in Safe Haskell

[Simon Peyton Jones]

|  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 at gmail.com [mailto:davidterei at gmail.com] On Behalf Of
|  David Terei
|  Sent: 10 April 2015 09:38
|  To: Simon Peyton Jones
|  Cc: Omari Norman; haskell Cafe; ghc-devs at 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 at 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 at 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?
|  >
|  >
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