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 .