Re: [GHC] #11632: Data.Char repeated readLitChar barfs on output from show "ó1"

[GHC]

#11632: Data.Char repeated readLitChar barfs on output from show "ó1"
-------------------------------------+-------------------------------------
        Reporter:  inversemot        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  7.10.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Linux             |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * keywords:   => newcomer


Comment:

 inversemot: what should `lexLitChar "\\243\\&1"` return in your opinion:
 1. unchanged
 `[("\\243","\\&1")]`
 2. consume the `\\&1`
 `[("\\243","")]`
 3. consume and include the `\\&1`
 `[("\\243\\&1","")]`

 I suppose option 2.

 For a newcomer: I think you'll want to either change the function
 `lexChar` in the file `libraries/base/Text/Read/Lex.hs`, and/or the
 functions `lexLitChar` and `readLitChar` in `libraries/base/GHC/Read.hs`.

 * Why not change the function `lexCharE`? Because it is used by the
 function `lexLitChar`, which lexes a character surrounded by single
 quotes, but `'x\&'` isn't a valid character (maybe it should be? It would
 simplify things.). Also note that the function `lexString` handles `\&` by
 itself in `lexEmpty`.

 Don't forget a [wiki:Building/RunningTests/Adding test] and
 [wiki:WorkingConventions/FixingBugs submit] your patch to Phabricator.


 For reference, the
 [https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-200002.6
 Haskell 2010 report] has this to say:

 > The escape character \& is provided as a “null character” to allow
 strings such as "\137\&9" and "\SO\&H" to be constructed (both of length
 two). Thus "\&" is equivalent to "" and the character '\&' is disallowed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11632#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler