#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: | ---------------------------------+------------------------------------------ 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler