Re: [Haskell-cafe] space leak when repeatedly calling Control.Monad.State.Strict.modify

Claude Heiland-Allen wrote:

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

Thanks! So, why does Control.Monad.State.Strict.modify not do that? And, I still don't quite understand why this only happened when the updated value is obtained using IO. -- see shy jo