[Haskell-cafe] Generalized Newtype Deriving not allowed in Safe Haskell

Simon Peyton Jones simonpj at microsoft.com
Thu Apr 9 07:56:53 UTC 2015

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?


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:


But this language on GNTD not being part of the safe language was removed in the 7.8 manual:


The GHC release notes don't say anything about this one way or the other.  Thoughts?
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