[GHC] #7881: Warning for pointless ranges like [5..2]

[GHC]

#7881: Warning for pointless ranges like [5..2]
---------------------------------+------------------------------------------
    Reporter:  mpe               |       Owner:                  
        Type:  feature request   |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.3           
    Keywords:  beginner,warning  |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Changes (by simonpj):

  * difficulty:  => Unknown


Comment:

 I think this is a good idea.  The gotcha is this:
 {{{
 newtype RInt = RI Int deriving( Eq, Show )
 instance Ord RInt where
   compare (RI a) (RI b) = compare b a
 instance Enum RInt where
   enumFromTo (RI a) (RI b) = map (RI . negate) [negate a .. negate b]
 instance Num RInt where
   fromInteger n = RI (fromInteger n)

 main = print ([3..1] :: [RInt])
 }}}
 Here `RInt` has an `eunuFromTo` implementation that doesn't produce an
 empty list!

 So you need to wait till after the type checker, and bleat only for empty
 ranges on known types like `Int`, `Integer`, `Char` and so on.

 Fortunately the desugarer, which runs afer the type checker, has the right
 plumbing to produce warnings.

 If someone wants to do this, I'm open to a patch.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7881#comment:1>
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