Hi, On 30/01/12 01:07, Joey Hess wrote:

The attached test case quickly chews up hundreds of MB of memory. If modified to call work' instead, it runs in constant space.

Somehow the value repeatedly read in from the file and stored in the state is leaking. Can anyone help me understand why?

Control.Monad.State.Strict is strict in the actions, but the state itself is still lazy, so you end up building a huge thunk in the state containing all the updates that ever took place to the initial state. Using this should fix it: modify' :: MonadState s m => (s -> s) -> m () modify' f = do x <- get put $! f x -- force the new state when storing it With the attached code, the first case (using modify) prints out a trace like: test work:1 modify work:2 modify work:3 modify work:4 modify work:5 modify work:6 modify work:7 modify work:8 modify work:9 modify work:10 modify update:vnbz update:dzgd update:hzla update:nudd update:bzfl update:muht update:hims update:jakj update:lvrt update:qdxo initial MyState {val = "vnbz"} Notice how the state updates are only evaluated right at the end, when the value is forced - note also that this means that all the data needs to hang around until then. The second case (using modify') forces the state as it goes along: test' work:1 modify' update:zwre initial work:2 modify' update:fefg work:3 modify' update:eoqa work:4 modify' update:xtak work:5 modify' update:tekd work:6 modify' update:qrsz work:7 modify' update:fdgj work:8 modify' update:alwj work:9 modify' update:kqsp work:10 modify' update:lazz MyState {val = "lazz"} Claude