Dear Haskellers, I am still trying to find out how to organize naturally a `lazy' printing of an accumulated data field. People advised me no-buffering and also the tilde usage. But there is some more obstackle. The `trace' is accumulated in fl1 by repeating consD, where data D = D {fl1 :: String, fl2 :: String} consD :: Char -> D -> D consD a d = d {fl1 = a:(fl1 d)} (tilde is not needed here?) The whole process computes res :: D in a certain complex recursive manner. One of the ways to use res is to print out the `trace' field: let res = (complexProcess ...) :: D in show $ fl1 res I need fl1 res to be formed `lazily'. For example, take 2 $ fl1 res may be ready (and shown) immediately, and last $ fl1 res may be ready long after. If it was [Char] istead of D, then this would be easy. Because head $ (a:) xs = a For example, head $ ('a':) $ (error "") = 'a' But is this true for consD and headD, where headD :: D -> Char headD = head . fl1 ? That is headD $ consD a res =?= a If res is not a bottom, then, yes. Right? And if res occurs, say, (error ""), then consD cannot intrude into fl1 res, and the whole expression has a value _|_. Right? So, generally, headD $ consD a res is not equivalent to a; hence the head members of fl1 res have to wait at least until it becomes clear that res is not a bottom. Right? So far, I see the two way-outs. 1. To insert the usage of the `trace' tool -- which does not look nice. 2. To have a result res :: [D] instead of D. And as soon as a new step appears, form res ++[d {fl1 = [step]}] where d = last res, fl2 also can be modified. Now, map fl1 res is the needed trace, which, I believe, would be formed `lazily'. I wonder a) whether there is a nicer solution, b) what happens if the Language treats the constructors as the homomorphisms. For example, head $ (a:) xs = a ==> head $ fl1 $ D {fl1 = (a:) fl1} = a Here (D, fl1) is a constructor, and its insertion between the functions head and (a:) preserves the above relation between them (is this called `bottom chasing' ?). Probably, this will eliminate unneeded strictness -- ? Thank you in advance for the explanations. Copy, please, the answer to mechvel@botik.ru ----------------- Serge Mechveliani mechvel@botik.ru