Hi,

I've started looking into using `Data.Coerce` (and the `Coercible` type-class) for a personal project and was wondering why coercing between `Int` and `Word` is not allowed? I don't see any fundamental reason why this shouldn't work...

Perhaps, it's just a matter of GHC's implementation details leaking out? IIRC internally GHC has separate `RuntimeRep`/`PrimRep` for a `Word#` and for an `Int#`. If that's the case, would it make sense to unify these? Their actual runtime representation should be the same and I'd expect most (all?) of their differences should be attached to `PrimOp`s.

And that leads me to another question--what exactly goes wrong here:

```

data Foo = Foo Int#
data Bar = Bar Int#

test :: Bar
test = coerce (Foo 42#)

```

Which fails with: "Couldn't match representation of type ‘Foo’ with that of ‘Bar’ arising from a use of ‘coerce’"

Thanks in advance!

- Michal

PS. The ability to coerce through things like lists is amazing :)