A while back I was playing with Data.Map was getting irritated about lookups that fail having the type of values, but wrapped in an extra monad. I decided to work around this by putting a default in the data type itself, so we have a "functional map" data FMap k a = FMap (k -> a) (Map k a) This has been really convenient - it's a monad, and failed lookups have the same type as successful ones. lookup :: (Ord k) => k -> FMap k a -> a lookup k (FMap f m)= Map.findWithDefault (f k) k m This also makes it much nicer to build a function that tabulates a list of pairs (nicer than I've found using Data.Map, anyway): fromPairs :: (Ord k) => [(k,a)] -> FMap k [a] fromPairs = foldl' (flip . uncurry $ insertWith (:)) $ return [] insertWith :: (Ord k) => (a -> b -> b) -> k -> a -> FMap k b -> FMap k b insertWith f k x m = case lookup k m of v -> insert k (f x v) m Ok, great, but fromPairs is blowing the stack. It does fine for a while, but today I was trying to use it for a few million pairs. It runs for a while, eats a couple gigs of ram, and then I get a stack overflow. Any advice for tracking this down? Thanks! Chad