[GHC] #11609: Document unicode report deviations

[GHC]

#11609: Document unicode report deviations
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           Reporter:  thomie         |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Documentation  |           Version:  7.10.3
           Keywords:  unicode,       |  Operating System:  Unknown/Multiple
  report-impact                      |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #1103, #4373,
                                     |  #10196, #11012
Differential Rev(s):                 |         Wiki Page:
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 @nomeata mentions in #10196:

 The report specifies “Haskell compilers are expected to make use of new
 versions of Unicode as they are made available.” So if we deviate from
 that, we should make sure that

 * the user’s guide explicitly lists all deviations from the report
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs-and-
 infelicities.html#infelicities-lexical in this section], and
 * that the Haskell prime committee is going to be aware of these
 (sensible) deviations, so that they can become official.

 Certain deviations are (there might be more):
 * `OtherLetter` are treated as lowercase (#1103), and thus allowed in
 identifiers.
 * `ModifierLetter` (#10196) and `OtherNumber` (#4373) are allowed in
 identifiers, but only starting from the second character.
 * `$decdigit = $ascdigit -- for now, should really be $digit (ToDo)` (see
 compiler/parser/Lexer.x)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11609>
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