On Feb 10, 2008 9:52 PM, Thomas Hartman wrote:

So, I would say this proves my main point, which was that you could accomplish the same thing using the writer monad that you could do using the more "ad hoc" trace function from Debug.Trace.

Not really. That only happens with your implementation of myfoldrD. If you write it as myfoldrD' f z [] = z myfoldrD' f z (x:xs) = x `f` trace ("x,r: " ++ (show (x,r))) r where r = myfoldrD' f z xs then we have the expected behavior *Main> myfoldrD (:) [] [1..5] x,r: (5,[]) x,r: (4,[5]) x,r: (3,[4,5]) x,r: (2,[3,4,5]) x,r: (1,[2,3,4,5]) [1,2,3,4,5] *Main> myfoldrD' (:) [] [1..5] [1x,r: (5,[]) x,r: (4,[5]) x,r: (3,[4,5]) x,r: (2,[3,4,5]) x,r: (1,[2,3,4,5]) ,2,3,4,5] *Main> myfoldrD const 0 [1..] Interrupted. *Main> myfoldrD' const 0 [1..] 1 *Main> myfoldrD (\x xs -> if x < 0 then [] else x:xs) [] ([1,2,3,-1] ++ repeat 0) *** Exception: stack overflow *Main> myfoldrD' (\x xs -> if x < 0 then [] else x:xs) [] ([1,2,3,-1] ++ repeat 0) [1x,r: (3,[]) x,r: (2,[3]) x,r: (1,[2,3]) ,2,3] As Debug.Trace hides the IO monad in a pure computation (i.e. unsafePerformIO) we can use it from the inside of the [pure] function that is passed to foldr. Note that we could also implement a Writer monad on top of unsafePerformIO, you basically just change Debug.Trace to an IO action that does the mappend as Writer would but in an IORef. In the end you read that IORef and do a big tell to the outside Writer monad. I'd say that this is a safe use of unsafePerformIO as it shouldn't break referential transparency. But without this hack I don't think we could do the same thing. Good news is that the hack is 'hideable' as are the hacks from ByteString, for example. Cheers, -- Felipe.