SerializedCompactPtr is defined as:

data SerializedCompact a = SerializedCompact
  { serializedCompactBlockList :: [(Ptr a, Word)]
  , serializedCompactRoot :: Ptr a
  }

But, these Ptr a values are a lie, because they don't point to something of type 'a', which makes the documentation for ghc-compact sort of confusing to look at. A more accurate type would just be Addr.

The consequences of this being changes to Addr are 1: breaking API changes (though not many people use compact regions) 2: A dependency on primitive would be necessary, though I'm again unsure how big of a deal this is, given that ghc-compact already depends on bytestring. (Addr should probably be moved to base, and re-exported from primitive, which would avoid this issue.)