David Place wrote:
On Jun 29, 2011, at 4:30 AM, Heinrich Apfelmus wrote:
It is still possible to write programs that return unexpected _|_, but they violate a clear conceptual guideline ("only fully evaluated values may escape the scope of withFile ").
I wonder. If that is the behavior you desire, why not just call deepseq on the value before you return it to guarantee it.
Hm. The idea was that while evaluating the return value in full is a sufficient requirement, it is not a necessary requirement. The return value may contain unevaluated expressions as long as the input stream is being forced. Example: do s <- hGetContents h let x = take 10 s `deepseq` [reverse . drop 3 $ take 7 s] evaluate x hClose h Evaluating x to weak head normal form will force the first 10 characters of the input, but x will still contain unevaluated expressions. (This is what Iteratees do: they force the input without forcing the return value.) But you are probably right, it's better to give guarantees. The behavior above can still be simulated with a broken deepseq instance. data Lazy a = Lazy a data NFData (Lazy a) where rnf _ = () -- broken on purpose This way, breaking the guarantee takes more effort than not breaking it, as it should be.
Granted, Iteratees make it impossible to write such programs, but they come with the terrible price of code duplication.
I don't really understand this objection, though. Won't any iteratee library provide all those functions? The user shouldn't have to write them. In this way, it is certainly no worse than the duplication brought about by ByteStrings. Also, isn't that the point of the ListLike class?
I think the ListLike class demonstrates very well that there will always be useful list functions that are not provided by an existing API. :) Also note that the ListLike class from the ListLike package doesn't work for Iteratees, they have to provide their own functions and so on. At some point, the count of different implementations for, holy lambda, *lazy lists* simply becomes ridiculous. ;) Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com