[GHC] #10152: Testsuite diffs can be misleading when normalisers are in use

[GHC]

#10152: Testsuite diffs can be misleading when normalisers are in use
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        Reporter:  ezyang            |                   Owner:
            Type:  task              |                  Status:  new
        Priority:  low               |               Milestone:
       Component:  Test Suite        |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by Thomas Miedema <thomasmiedema@…>):

 In [changeset:"5ddd90415f307cac72d75d86da58e552b168ee30/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5ddd90415f307cac72d75d86da58e552b168ee30"
 Testsuite: diff non-whitespace normalised output (#10152)

 On a test failure, we show a diff between the expected and the actual
 output. The method of how we do this has changed a couple of times:

 * In 2007: 9951189ccf90b709436fa55ee49eeef031f79f4e
   "On failure, diff the normalised test outputs"

 * In 2011: 3019b1e409c129ef7af63e6a7408fb36ec44444b
   "When the output files differ, present the diffs between the *actual*
   output, not the normalised output. The latter may have newlines
   removed, making the diff unreadable."

 * In 2015 (now): do something in between.
   - Do apply the normalisers again, to make the diff smaller (only
     showing the actual problem).
   - But don't apply normalise_whitespace, as it indeed makes the diff
     unreadable.

 Differential Revision: https://phabricator.haskell.org/D984
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10152#comment:1>
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