Depending on the context, it may or may not be wise to distinguish
between undefined and [undefined]. This is a matter of strictness,
laziness, and totality. If you identify all bottoms as one, you
essentially restrict yourself to (what might as well be, for the
purposes of this discussion) a strict subset of the Haskell language.
If you distinguish between values that "contain" bottom and "are"
bottom, then you need to deal with the semantics of laziness.
Like I said, it is classically valid, so choosing these semantics
means that it will be safe. But the undecidability of comparing
distinct bottoms means that you have to make a choice.
On 7/24/12, Christian Sternagel
It's a classically valid inference, so you're "safe" in that respect, and it is true that evaluating x == y requires traversing x and y, so that if x or y "are" bottom, (x == y) and (y == x) will both be bottom. Well, (x == y) could result in bottom, even if neither x nor y are bottom, e.g., [undefined] == [undefined]. -cheers chris