Example.hs

newtype AtomP = AtomP Word64

view :: AtomP -> Maybe (Bool, Word64)
view x = let mask = complement (bit 63 .|. bit 62)
             raw = coerce x in
              if | testBit raw 63 -> Nothing
                 | testBit raw 62 -> Just (False, pext64 raw mask)
                 | otherwise -> Just (True, pext64 raw mask)
                 
-- I want to expose a view of AtomP as a Maybe (Bool, Word64) on the basis of the function above.
-- I want the view to be read-only (so for pattern matching).

pattern Atom :: Bool -> Word64 -> AtomP
pattern Atom x y <- (view->Just (x, y))