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

[GHC]

#14364: Reduce repetition in derived Read instances
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        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:                    |
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Comment (by tdammers):

 Replying to [ticket:14364 bgamari]:

 > Let's factor this pattern out into a `readField` helper in `GHC.Read`,
 > {{{#!hs
 > readField :: String -> ReadPrec a -> ReadPrec a
 > readField fieldName readVal = do
 >     expectP  (Ident fieldName)
 >     expectP (Punc "=")
 >     readVal
 > {-# NOINLINE readField #-}
 > }}}
 > This will at least knock off a constant factor from the size of what
 should not be performance-critical code.

 The `readField` function actually has to be a bit more complex than that
 to cater for symbol-named fields (e.g. `(#)`). We can however make the
 decision at compile time, because we already know the label.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14364#comment:5>
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