Generalized Newtype Deriving not allowed in Safe Haskell

[David Terei]

On 10 April 2015 at 01:48, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> |  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.

Yes, so as you point out in the paper, this is done to preserve
abstractions, but the same rule isn't applied to data types since some
types like IORef don't even have constructors that can be in scope.

Ideally I'd like to find a way forward that works for everyone and
isn't just a Safe Haskell mode setting.

I think the first question is, are there situations where you'd want
to use `coerce` internally to a module but disallow it externally? The
role mechanism is a little awkward as it doesn't allow this (although
it does for newtype's). If yes, then I think we should start there.

If it seems we don't need external vs internal control, then we could
simply change the default to be that GHC sets referential type
parameters to nominal and allows them to be weakened to referential
through role annotations. We could use hackage to test how much
breakage this would cause.

The third option is something Safe Haskell specific, so probably
applying the newtype constructor rule to data types.

>
> 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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