The QuickCheck property now succeeds:

prop :: Fixed B7 -> Bool
prop a = read (show a) == a

This changes the Show instance for Fixed to round up, rather than down
when calculating a digit. This needs to happen because Read also
rounds down:

data B7

instance HasResolution B7 where
  resolution _ = 128

1 / 128 = 0.0078125

read "0.007" = (0.000 :: Fixed B7)

Here is an example of the change to Show:

showFixed False (0.009 :: Fixed B7)

-- Broken: "0.007"
-- Fixed:  "0.008"

And now Read can continue to round down:

read "0.008" = (0.0078125 :: Fixed B7)

QuickCheck from GHCi