[GHC] #11527: Pattern match translation suboptimal

[GHC]

#11527: Pattern match translation suboptimal
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        Reporter:  augustss          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.3
      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):

 For `Integer` I agree we could and should do better.  But what about
 {{{
 baz :: (Num a) => a -> Int -> Int
 }}}
 which is what you'll get if you don't have a type signature.

 The report specifies something like this:
 {{{
 baz n x
  | n == fromInteger 10, x == 1 = 1
  | n == fromInteger 20, x == 1 = 2
  | n == fromInteger 10, x == 2 = 2
 ...etc...
 }}}
 Now you might think that we can common up all those tests for `n ==
 fromInteger 10` to get
 {{{
 baz n x
   | n == fromInteger 10
   = case x of
       1 -> 1
       2 -> 2
       ... etc ...

   | n == fromInteger 20
   = ...etc..
 }}}
 But what if both `n == fromInteger 10` and `n == fromInteger 20` are
 ''both true''? Now it's not OK to group together all the matches against
 10.  How annoying!

 The best we can do is to do CSE on those `fromInteger` calls.

 It's all very irritating but I don't see how to do better.  Except for
 `Integer` when we know that the `Num` instance has good properties.  Is
 that particular case worth improving?  Perhaps, and it would not be hard
 -- I can offer guidance.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11527#comment:1>
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