I'm facing the dark side of lazyness recently. Typical pattern is like this. My code was working fine and I was happy. I just wanted to inspect some properties of my code so I made a slight chane go the code such as adding counter argument or attaching axulary data filed to original data for tracing how the data has been constructed. All of a sudden the program runs out of memory or overflows the stack. One problem is that it comes up unexptectedly. Another even worse problem is that sometimes I get no idea for the exact loation causing the leak! It really panics facing such darkness of lazy evaluation. Just a small innocent looking fix for inspection or tracing blow things up, sometime with no clue for its reason. When I put a debugging or tracing operating in my software that can be toggled, how could I be sure that turning on those features won't crash my software written in Haskell? Are there appraoches to detect and fix these kind of problem? Haskell may be type safe but not safe at all from unexpteded diversion, not because of the programmers' mistake but just because of the lazyness. I have posted an wiki article including one example of this dark side of lazyness I encountered when I tried to count the number of basic operations in sorting algorithm. This was a rather simple situation and we figured out how to cure this by self equality check ( x==x ) forcing evaluation. There are wose cases not being able to figure out the cure. I wrote a fucntion for analyzing some property of a graph, which worked fine. fixOnBy t p f x = if t x' `p` t x then x else fixOnBy t p f x' where x' = f x fixSize f x = fixOnBy Set.size (==) f x sctAnal gs = null cgs || all (not . null) dcs where gs' = fixSize compose $ Set.fromList [(x,y,cs) | To _ x y cs<-Set.toList gs] cgs = [z | z@(x,y,cs)<-Set.toList gs', x==y] dcs = [ [c| c@(a,D,b)<-Set.toList cs , a==b] | (_,_,cs)<-cgs] compose gs = trace ("## "++show (Set.size gs)) $ foldr Set.insert gs $ do (x1,y1,cs1) <- Set.toList gs (_,y2,cs2) <- takeWhileFst y1 $ Set.toList $ setGT (y1,Al""(-1),Set.empty) gs return (x1,y2,cs1 `comp` cs2) takeWhileFst y = takeWhile (\(y',_,_) -> y==y') This fucntion makes a transitive closure of the given set of relations by fixpoint iteration on the size of the set of weighted edegs. Sample output is like this. *Main> main ## 170 ## 400 ## 1167 ## 2249 ## 2314 False When I add an extra data field for tracing how the new item was made (e.g. tag [a,b,c] on a->c if it was generated by a->b and b->c) It suddenly overflows the stack even before printing out the trace. The following is the code that leaks memory. sctAnal gs = null cgs || all (not . null) dcs where gs' = fixSize compose $ Set.fromList [TT (x,y,cs) [] | To _ x y cs<-Set.toList gs] cgs = [z | z@(TT (x,y,cs) _)<-Set.toList gs', x==y] dcs = [[c| c@(a,D,b)<-Set.toList cs , a==b] | TT (_,_,cs) _<-cgs] compose gs = trace ("## "++show (Set.size gs)) $ foldr checkInsert gs $ do TT (x1,y1,cs1) l1 <- Set.toList gs TT (_,y2,cs2) l2 <- takeWhileTTfrom y1 . Set.toList $ setGT (TT (y1,Al""(-1),Set.empty) []) gs return $ TT (x1,y2,cs1 `comp` cs2) (l1++y1:l2) takeWhileTTfrom y = takeWhile (\(TT (y',_,_) _) -> y==y') checkInsert x s | Set.member x s = s | otherwise = Set.insert x s data TT a b = TT a b deriving (Show) instance (Eq a, Eq b) => Eq (TT a b) where (TT x lx) == (TT y ly) = lx==lx && ly==ly && x == y instance (Ord a, Ord b) => Ord (TT a b) where (TT x lx) < (TT y ly) = lx==lx && ly==ly && x < y The really intersting thing happens when I just make the Ord derived the stack does not overflow and starts to print out the trace. (It is not the result that I want though. My intention is to ignore the tags in the set operation) data TT a b = TT a b deriving (Show,Eq,Ord) I believe my Eq and Ord instances are even more stricter than the derived ones. Is there some magic in "deriving" that prevents memory leak? I've even followed the instance declaration that would be the same as deriving but the that leaks memory. data TT a b = TT a b deriving (Show) instance (Eq a, Eq b) => Eq (TT a b) where (TT x lx) == (TT y ly) = x == y && lx == ly instance (Ord a, Ord b) => Ord (TT a b) where (TT x lx) < (TT y ly) = x < y || x == y && lx < ly This is really a panic. -- Ahn, Ki Yung