Re: [GHC] #9240: "read . show ≡ id" not satisfied by Data.Fixed

[GHC]

#9240: "read . show ≡ id" not satisfied by Data.Fixed
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              Reporter:  hvr         |            Owner:  ekmett
                  Type:  bug         |           Status:  patch
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.2.1
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #9231
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D547   |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"7c38e985aa211ca44039c6d1db9fa13690749c59/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="7c38e985aa211ca44039c6d1db9fa13690749c59"
 Make `read . show = id` for Data.Fixed (fix #9240)

 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
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9240#comment:4>
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