Are safe coercions safe in the sense of Safe Haskell?

[Wolfgang Jeltsch]

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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>