[GHC] #8782: Using GADT's to maintain invariant in GHC libraries

[GHC]

#8782: Using GADT's to maintain invariant in GHC libraries
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              Reporter:              |            Owner:
  Iceland_jack                       |           Status:  infoneeded
                  Type:  task        |        Milestone:  7.10.1
              Priority:  lowest      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by Iceland_jack):

 I removed some exported functions like `mapOL` (that only appears in a
 comment) and `foldrOL` and `foldlOL` (replacing occurrences with
 `Data.Foldable.foldr` and `Data.Foldable.foldl`.

 Other functions like (`concatOL`, `appOL`, `fromOL`, ...) could as well be
 dropped in favour of the `Monoid` and `IsList` methods in the future, but
 they are used substantially more than the ones I replaced so I decided
 against it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8782#comment:4>
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