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

[Claude Heiland-Allen]

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
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