[GHC] #16364: Derived Enum for small number of constructors seems suboptimal

[GHC]

#16364: Derived Enum for small number of constructors seems suboptimal
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        Reporter:  Fuuzetsu          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I agree that a straight pattern-match on the tag might be better, at least
 for small enumerations.

 Another interesting question is this: what is the behaviour of the
 primitive operation `tagToEnum#` when given an out-of-range tag?  Does it
 crash somehow, return a bogus answer, or call `error`?   It think it might
 be the former, relying on the caller to check for out-of-range cases, like
 array indexing.  We should document this properly.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16364#comment:1>
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