[GHC] #10344: Make BranchList simpler

[GHC]

#10344: Make BranchList simpler
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              Reporter:  goldfire    |             Owner:  goldfire
                  Type:  task        |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 As Adam Gundry suggested, `BranchList` could be made much simpler by
 actually storing a list in the `Branched` case.

 Though, as I'm thinking about it, what's better about a real list over a
 `BranchList`? We're more familiar with it... but perhaps adding a
 `Foldable` instance for `BranchList` would solve the problem.

 In any case, this ticket is to record the desire for improvement. If Adam
 is up to it, he may fix this with Phab:D841. Otherwise, I'll get to it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10344>
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