Warning for pointless ranges like [5..2]

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 [].

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Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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