On 08/10/2012 20:11, Mikhail Glushenkov wrote:
Hello,
It's a relatively well-known fact that GHC allows for multiple type class instances for the same type to coexist in a single program. This can be used, for example, to construct values of the type Data.Set.Set that violate the data structure invariant. I was mildly surprised to find out that this works even when Safe Haskell is turned on:
https://gist.github.com/3854294
Note that the warnings tell us that both instances are "[safe]" which gives a false sense of security.
I couldn't find anything on the interplay between orphan instances and Safe Haskell both in the Haskell'12 paper and online. Is this something that the authors of Safe Haskell are aware of/are intending to fix?
A fine point. Arguably this violates the module abstraction guarantee, because you are able to discover something about the implementation of Set by violating its assumption that the Ord instance for a given type is always the same. I don't know what we should do about this. Disallowing orphan instances seems a bit heavy-handed. David, Simon, any thoughts? (can someone forward this to David Mazieres? all the email addresses I have for him seem to have expired :-) Cheers, Simon