• puffnfresh's avatar
    Make `read . show = id` for Data.Fixed (fix #9240) · 7c38e985
    puffnfresh authored
    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)
    
    Reviewed By: hvr, ekmett
    
    Differential Revision: https://phabricator.haskell.org/D547
    7c38e985
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