[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
      Resolution:                    |                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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Comment (by goldfire):

 Replying to [comment:2 simonpj]:
 > Yes yes!  Simple is good!  Needs a comment to stress that in the
 `BranchedList` case the list may legitimately be empty.

 But a `BranchList Branched` currently still can't be empty -- Adam's empty
 CTF patch uses the `Maybe (CoAxiom Branched)` approach to deal with empty
 CTFs. Consumers of `fromBranchList` assume they're getting a non-empty
 list.

 So, we could do

 {{{
 data BranchList a br where
   OneBranch :: a -> BranchList a Unbranched
   ManyBranches :: a -> [a] -> BranchList a Branched -- requires at least
 one element
 }}}

 This is certainly simpler than the current story.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10344#comment:3>
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