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

[GHC]

#7881: Warning for pointless ranges like [5..2]
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Reporter:  mpe               |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.3             |       Keywords:  beginner,warning
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 Haskell beginners sometimes tend to write ranges like [3..1] or [5..2] and
 assume that the result will be [3,2,1] or [5,4,3,2]. This is not the case
 of course.

 I suggest that literals like [5..2] generate a warning at compile time,
 analogous to:

 "Foo.hs:32: Warning: Literal list [5..2] evaluates to [] because 5 > 2 and
 the default step size is +1. Replace the literal with the empty list or
 with [5,4..2] to suppress this warning."

 In my opinion this should only be a compile time warning, not a runtime
 warning. If a > 3 and b is < 3 and you write [a..b] the result is still
 [], but that mistake can only be caught at runtime, and requires an
 additional run-time check. In some cases it may even be the desired
 behaviour to let [a..b] evaluate to [] if a>b. On the other hand, a
 literal expression like [5..2] is most likely an error, because if the
 user would expect [5..2] to evaluate to [], he would simply write [].

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