[GHC] #14364: Reduce repetition in derived Read instances

[GHC]

#14364: Reduce repetition in derived Read instances
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10980 #7258      |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"dbd81f7e86514498218572b9d978373b1699cc5b/ghc" dbd81f7/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="dbd81f7e86514498218572b9d978373b1699cc5b"
 Factor out readField (#14364)

 Improves compiler performance of deriving Read instances, as suggested
 in the issue.

 Additionally, we introduce `readSymField`, a companion to `readField`
 that parses symbol-type fields (where the field name is a symbol, e.g.
 `(#)`, rather than an alphanumeric identifier. The decision between
 these two functions is made a compile time, because we already know
 which one we need based on the field name.

 Reviewers: austin, hvr, bgamari, RyanGlScott

 Reviewed By: bgamari

 Subscribers: RyanGlScott, rwbarton, thomie

 Differential Revision: https://phabricator.haskell.org/D4108
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14364#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler