[Haskell-cafe] Re: Control.Exception.evaluate - 'correct definition' not so correct

Abhay Parvate wrote:

Just for curiocity, is there a practically useful computation that uses 'seq' in an essential manner, i.e. apart from the efficiency reasons?

I don't think so because you can always replace seq with const id . In fact, doing so will get you "more" results, i.e. a computation that did not terminate may do so now. In other words, we have seq _|_ = _|_ seq x = id for x > _|_ but (const id) _|_ = id (const id) x = id for x > _|_ So, (const id) is always more defined (">") than seq . For more about _|_ and the semantic approximation order, see http://en.wikibooks.org/wiki/Haskell/Denotational_semantics Regards, apfelmus