Hello again, it's me, your favourite haskell beginner :) This one really has me puzzled. I've got a little example here - the program itself is a bit silly but it illustrates my problem nicely. He's a little program: import qualified Data.ByteString as B chunkFiller i bs = do if (B.null bs ) then return 1 else chunkFiller (i+1) bs main = do let bs = (B.singleton 10) j <- chunkFiller 0 bs return 1 It runs forever, which is fine, but it doesn't run in constant space. In fact, it gobbles up all the memory until it can't get any more and dies. I can't for the life of me work out what is going on, surely there is some enormous thunk being built up somewhere, but for all the Debug.Trace(), $!, seq's, and profilign outputs in the world I can't figure out where. What is *really* interesting is that if you replace the 'if' line with if (B.null bs || i== -1) then return 1 (i never equals -1) then it runs in constant space! That just boggles my mind. As usual, any thoughts greatly appreciated. All the best, Philip