unsafeCoerce and type aliases under type applications

[Erik Hesselink]

On Sun, Dec 4, 2011 at 15:04, Brent Yorgey <byorgey at seas.upenn.edu> wrote:
> On Sun, Dec 04, 2011 at 11:17:25AM +0100, Joachim Breitner wrote:
>> Hi,
>>
>> Am Sonntag, den 04.12.2011, 00:50 -0500 schrieb wren ng thornton:
>> > On 12/3/11 9:07 PM, Felipe Almeida Lessa wrote:
>> > > On Sat, Dec 3, 2011 at 5:39 PM, Joachim Breitner
>> > > <mail at joachim-breitner.de>  wrote:
>> > >> have used unsafeCoerce to change the type inside a container to a "type"
>> > >> alias in real code, but your post makes me wonder: Under what
>> > >> circumstances is that safe? Is that documented somehow? Can a tool or
>> > >> the compiler decide for us whether it is safe?
>> > >
>> > > AFAIK, newtypes are safe, and for everything else you're on your own.  =)
>> >
>> > N.B., newtypes are safe in the sense of congruent rewriting; i.e., if X
>> > is a newtype of Y, then we can rewrite X to Y (or Y to X) in any subterm
>> > of the type term (just like if X = Y or X ~ Y). It's not just at the
>> > top-level of the type term.
>>
>> that is what I would expect at first glance, but at least some type
>> features break this behavior:
>>
>> So the question remains: Under which circumstances is newtypes coercing
>> within a type term using unsafeCoerce safe?
>
> Intuitively, it is safe to do newtype coercing as long as the newtype
> is treated "parametrically" by the context, i.e. it never appears as
> the argument to a type family. In principle this analysis could be
> done in an automated way; actually, the fact that GHC *doesn't* do
> this analysis means that GeneralizedNewtypeDeriving is unsound in the
> presence of type families; see
> http://hackage.haskell.org/trac/ghc/ticket/1496.

It's not just type families though, right? I mean, a (Map A) cannot be
coerced to a (Map B), even if B is a newtype over A, since they might
have different Ord instances, and thus a different map structure.

Erik