John Meacham wrote:
things like DeepSeq and Typeable will most likely have optimized versions on various compilers which is why I'd like to see the restriction that the only way to create instances for these two classes is via the "deriving" mechanism of the compiler.
Please make it so! Allowing users to create their own instances of Typeable is tantamount to allowing them to define the _total_ function unsafeCoerce :: (Typable a, Typeable b) => a -> b That function always succeeds; you don't want to trust its result in general though. If we add -fallow-incoherent-instances, then the Typeable constraint disappears and we obtain the total function of the type a->b. The fact these functions are expressible in Haskell at all is quite disconcerting.