[GHC] #12206: No exposed API to get full text of ErrMsg

[GHC]

#12206: No exposed API to get full text of ErrMsg
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           Reporter:  bit            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHC API        |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In GHC version 7.10.3 it was possible to get the full text of an error
 message by using the functions `ErrUtils.errMsgShortDoc` and
 `ErrUtils.errMsgExtraInfo`.

 But in GHC version 8.0.1 the code has changed. The full text of the error
 message is stored in the `ErrMsg` field `ErrUtils.errMsgDoc`, but this
 field is hidden, and there doesn't seem to be any API to access its
 contents.

 The only workaround I can find is to use `ErrUtils.pprLocErrMsg`, and then
 strip out the first line containing the location info, and then manually
 unindent the 4-space "hang". This is quite ugly.

 I suggest exporting `ErrUtils.errMsgDoc`, and then for that to be useful
 the fields of `ErrUtils.ErrDoc` will also be needed. Additionally,
 `ErrUtils.formatErrDoc` could be a nice bonus to have (and if exported
 would lessen the need for the `ErrDoc` fields).

 The GHC API changes on every GHC release, so since backwards compatibility
 is not an issue, it would seem that there is no need to have a carefully
 exposed API, and there is no danger in liberally exporting stuff that
 clients could potentially find useful.

 Thank you

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12206>
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