In GHC.Ptr we see

type role Ptr representational
data Ptr a = Ptr Addr# deriving (Eq, Ord)

with no comments. Why is Ptr representational?

In the same module we have castPtr:

castPtr :: Ptr a -> Ptr b
castPtr (Ptr addr) = Ptr addr

which unpacks and repacks a Ptr. If Ptr was phantom, we could use coerce. And that in turn would actually make a lot of code more efficient – there are lots of calls to castPtr. Specifically, in nofib, I tried implementing castPtr with unsafeCoerce. Then I found:

• 12% less allocation in reverse-complem
• 7.3% less allocation in fasta.
• Binary sizes fell 0.1%.

Both these benchmarks are ones that do a lot of I/O, and it turns out that GHC.IO.Handle.Text.$wa1 has a castPtr that (all by itself) accounts for 12% of reverse-complem's total allocation! So making castPtr free is good.