Re: [Haskell-cafe] Daunting heap profile when using (<>)

14 Nov 2015

      On Sat, Nov 14, 2015 at 01:28:58PM +0100, martin wrote:
...
newtype Condition a  = Cnd {checkCnd :: a ->  (Bool, Condition a)}
-- | Create a 'Condition' from two conditions which holds when one of
-- them holds.
cor :: Condition a -> Condition a -> Condition a
cor c1 c2 = Cnd cnd
        where
            cnd a = let (b1', c1') = checkCnd c1 a
                        (b2', c2') = checkCnd c2 a
                    in if b1' || b2'
                       then (True,  cor c1' c2')
                       else (False, cor c1 c2)
logWhen :: Monoid log => Condition (Timed evt,dom) -> Logger evt dom log -> Logger evt dom log
logWhen cnd lgrIn = Lgr lgr'
        where
            lgr' tev dom =
                    case checkCnd cnd (tev, dom) of
                        (True, cnd') -> let (log', lgrIn') = runLogger lgrIn tev dom
                                         in (log', logWhen cnd' lgrIn')
                        (False,cnd')   -> (mempty, logWhen cnd' lgrIn)
Am I holding on to some data where I shouldn't? I cannot see it.
Sure, this

    (log', lgrIn') = runLogger lgrIn tev dom

allocates a thunk for log' that holds on to tev and dom.  Even if they are
small, holding onto them for thousands or millions of iterations is going to
leak a lot of space.

The only way to avoid this is to force log' sufficiently far that tev and
dom can be released.

Tom

Re: [Haskell-cafe] Daunting heap profile when using (<>)

Tom Ellis