Are safe coercions safe in the sense of Safe Haskell?

[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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>>> Glasgow-haskell-users at haskell.org
>>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>>> 
>> 
> 
> 
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