This is a small example of problem I described (don't ask me what it does): foo :: (() -> () -> [()] -> [()]) -> () -> [()] -> [()] foo k = \_ xs -> concatMap ($ [head xs]) [bar] where bar = let k' = k undefined undefined in \xs -> let k'' = [k' xs] in (() : (foldr1 (>>) k'')) k = foo (\_ -> k) --k a = foo (\_ -> k) a r = k undefined [] !! 4000 main = print r It is very sensitive to small changes, so be sure to use a high-quality cut-n-paste implementation. Basically, this code has a memory leak through xs; the xs passed to k' comes from the xs passed to foo, and is never forced, so a chain of thunks is built. If you run the code as is, either with time or heap profiling, you will see everything charged to k. If you instead use the commented version of k, you will see everything charged to foo, as expected. The command I run to test this is ghc --make -prof -auto-all Logic.hs && ./a.out +RTS -hc && hp2ps -c a.out.hp I'm using ghc 6.4.1 from Debian unstable. I'll copy this into a bug report too. I can try to explain what I did to arrive at this code if it wolud help, but it's a long story.... Andrew