Am Mittwoch, 27. August 2008 16:42 schrieb Tillmann Rendel:
Daniel Fischer wrote:
I don't have a CS degree either, so someone please correct me if I'm wrong (or confirm if I'm right). In Haskell, reducing a term to whnf (which e.g. seq is for) means evaluating it far enough that its top level constructor is known (or it's found to be bottom). For some types like Int, that means complete evaluation, for others it's far less. A list is in whnf if you know whether it's bottom, [] or h:t, where you might or might not know something about h and t. A Maybe term is in whnf if you know if it's bottom, Nothing or Just whatever. A function is in whnf if you know if it's bottom or \x -> rhs.
That's correct, but be careful with your use of the term bottom. Most bottoms are never "found to be bottom", but instead, the program runs forever trying to find the top-level constructor.
Distinguish between findable and unfindable bottoms? The difference in practice is clear, but is there also a theoretical difference?
Another thing: you do not need seq to evaluate something to whnf, you can just pattern match on the top-level constructor instead.
Yes, what I wanted to convey was that seq doesn't fully evaluate its first argument but only reduces it to whnf. Failed on that point :(
seq is there for polymorphic values where you cannot pattern match since you don't know the possible constructors. Pattern matching is the main mechanism in Haskell to trigger evaluation.
Tillmann
Cheers, Daniel