Are safe coercions safe in the sense of Safe Haskell?

[Wolfgang Jeltsch]

Hi,

the GHC 7.8.2 installation on my machine refuses to import GHC.Prim if
Safe is enabled. The locally generated documentation still claims that
GHC.Prim is Safe-Inferred, though. So this seems to be indeed a
documentation bug.

All the best,
Wolfgang

Am Sonntag, den 17.08.2014, 11:26 -0400 schrieb Richard Eisenberg:
> Have you tried doing this? If so, `coerce` is the least of our
> problems: `unsafeCoerce#` is much worse! When I just tried, GHC told
> me that I couldn't import GHC.Prim into a module with -XSafe enabled.
> 
> So, this seems to be a documentation bug (the Haddock description of
> GHC.Prim indeed says "Safe Inferred"), but not a "real" bug.
> 
> Let me know if you see otherwise!
> 
> Thanks,
> Richard
> 
> On Aug 16, 2014, at 6:06 PM, Wolfgang Jeltsch <g9ks157k at acme.softbase.org> wrote:
> 
> > Hi,
> > 
> > thank you for these links.
> > 
> > Still, it is interesting that also in GHC 7.8 you can have a coerce that
> > is considered “Safe”, although the discussions on Trac concluded that
> > this should not be the case. You can just import coerce via GHC.Prim,
> > which is “Safe-Inferred”.
> > 
> > All the best,
> > Wolfgang
> > 
> > Am Freitag, den 15.08.2014, 19:40 -0400 schrieb Richard Eisenberg:
> >> See https://ghc.haskell.org/trac/ghc/ticket/8745 and https://ghc.haskell.org/trac/ghc/ticket/8827 which discuss this problem at length.
> >> 
> >> The short answer: It's conceivable that a role-unaware library author would have abstraction expectations that are defeated through the use of `coerce`.
> >> 
> >> I would strongly welcome a proposal for how to make `coerce`, and hence GeneralizedNewtypeDeriving, to be considered Safe for 7.10.
> >> 
> >> Richard
> >> 
> >> On Aug 15, 2014, at 4:04 PM, Wolfgang Jeltsch <g9ks157k at acme.softbase.org> wrote:
> >> 
> >>> Hi,
> >>> 
> >>> I would expect the function
> >>> 
> >>>   coerce :: Coercible a b => a -> b
> >>> 
> >>> to be safe in the sense of Safe Haskell. However, the Data.Coerce module
> >>> is marked “Unsafe”. The coerce function is also available via GHC.Exts
> >>> and GHC.Prim. The former module is marked “Unsafe”, but the latter is
> >>> (surprisingly) marked “Safe-Inferred”.
> >>> 
> >>> What are the reasons behind this?
> >>> 
> >>> All the best,
> >>> Wolfgang
> >>> 
> >>> _______________________________________________
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> >>> 
> >> 
> > 
> > 
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>