[Haskell-beginners] Laziness problem, some code never being executed, when using Control.Monad.State.Strict

[Olivier Boudry]

Hi,

I found the solution to the problem. It was in the function calling
execCard where I was overwriting a state value with an older result.

execChance = do
    m <- get
    let c = ch m
    c' <- execCard c
    put m {ch = c'}      <-- overwriting the m value put in the execCard
function

I fixed it by adding one more get before the last put.

Thanks,

Olivier.


On Sat, Jan 26, 2013 at 5:29 PM, Olivier Boudry <olivier.boudry at gmail.com>wrote:

> Hi folks,
>
> I'm using the state monad (Control.Monad.State.Strict) to keep track of
> the state of a monopoly game (Project Euler problem #84). I'm really close
> to get something that works, but I just have some code with side effects
> only that does not execute and I struggle force it to execute. I used hpc
> to discover which part of the code was never run and found lines 93 to 95.
> In those lines I'm trying to update a visits Map that holds the number of
> times I've been on a particular cell of the board. What I don't understand
> is that the code in lines 66 to 68 is almost identical and this one gets
> executed. The visit Map is used at the very end to retrieve results.
>
>    57 execAction :: State Monopoly ()   58 execAction = do   59     m <- get   60     let b = board m   61     let s = CL.value b   62     case s of   63         (CC _) -> execCommunityChest   64         (CH _) -> execChance   65         G2J    -> execMove gotoJail   66         _      -> do   67             let v = insertWith (+) s 1 (visit m)   68             put m {visit = v}...
>
>    80     let c = cc m   81     c' <- execCard c   82     put m {cc = c'}   83    84 execCard :: CardsDeck -> State Monopoly CardsDeck   85 execCard cd = do   86     let cd' = CL.next cd   87     case CL.value cd of   88         Just mv -> do   89             execMove mv   90             return cd'   91         Nothing -> do   92             m <- get   93             let s = CL.value $ board m   94             let v = insertWith (+) s 1 (visit m)   95             put m {visit = v}   96             return cd'
>
> I tried to play with some $! to force execution but it never goes deep
> enough and I don't really know where to place these. I did not copy the
> whole solution as requested on the Project Euler site.
>
> Any help on this would be greatly appreciated,
>
> Thanks,
>
> Olivier.
>
>
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