Andy Gill wrote: > > On Apr 4, 2006, at 3:47 AM, Simon Marlow wrote: > >> On 30 March 2006 23:12, Andy Gill wrote: >> >>> Implementation: >>> >>> deepSeq (RAW_CONS <is_deep_seq'd_bit> ... fields ) = >>> if <is_deep_seq'd_bit> == True >>> then return /* hey, we've already deepSeq'd this */ >>> else set <is_deep_seq'd_bit> to True. >>> deepSeq (field_1) >>> ... >>> deepSeq (field_n) >>> deepSEQ (REF/MVAR...) = return >> >> So deepSeq doesn't return _|_ when passed a cyclic structure? This is a >> bad idea, because it lets you distinguish cyclic structures from >> infinite ones. deepSeq has to behave like a function, regardless of its >> implementation. >> >> Cheers, >> Simon > > Good observation, though pragmatically I'd rather the deepSeq to > behave well on loops. Its the thunks I'm trying to remove, not > the loop itself. > > Allowing loops in the returned value gives the the beauty of laziness > to construct the cycle, but the assurance that the structure does not > contain > thunks. A nice property, and a way to interact with laziness. > > let xs' () = 1 : 2 : xs' () > let xs2 = xs' > > let xs = 1 : 2 : xs > > So deepSeq xs2 ==> _|_, but deepSeq xs ==> xs > > I appeal to the "morally correct reasoning" argument .. If the program > terminates, then it is still correct. The deepSeq is an assertion about > the ability to represent the result in finite space.
I'm not convinced Simon's argument holds, as I don't think you can use deepSeq to write a Haskell function that will distinguish cyclic structures from infinite ones. If we can't do that, then we haven't really added any new semantic observational capability to the theory, so I think the "morally correct reasoning" argument holds. Simon? A -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Connections Inc. Fax: 503.350.0833 12725 SW Millikan Way, Suite #290 http://www.galois.com Beaverton, OR 97005 [EMAIL PROTECTED] _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://haskell.org/mailman/listinfo/haskell-prime
