[Haskell-cafe] I don't understand how ST works

[Nicu Ionita]

Hi,

I created a gist with a minimal (still 111 lines) module:

https://gist.github.com/2898128

I still get the errors:

WhatsWrong.hs:53:5:
     Couldn't match type `s' with `PrimState (ST s)'
       `s' is a rigid type variable bound by
           a type expected by the context: ST s [Move] at 
WhatsWrong.hs:48:21
     In a stmt of a 'do' block: listMoves ml
     In the second argument of `($)', namely
       `do { v <- U.new maxMovesPerPos;
             let ml = ...;
             listMoves ml }'
     In the expression:
       runST
       $ do { v <- U.new maxMovesPerPos;
              let ml = ...;
              listMoves ml }

WhatsWrong.hs:65:44:
     Couldn't match type `s' with `PrimState (ST s)'
       `s' is a rigid type variable bound by
           the type signature for nextPhaseOnlyCapts :: GenPhase s
           at WhatsWrong.hs:64:1
     Expected type: U.MVector (PrimState (ST s)) Move
       Actual type: U.MVector s Move
     In the return type of a call of `mlVec'
     In the third argument of `genCapts', namely `(mlVec ml)'

Thanks,
Nicu

Am 08.06.2012 02:47, schrieb Silvio Frischknecht:
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>> Now comes my question: in the impure values there is always that
>> "s". I was thinking that the whole structure should have s as a
>> parameter:
> Yes
>
>>> data MList s = MList { mlVec :: MVector s Move, mlNextPh :: MList
>>> ->
>> ST s (Maybe (MList s)) }
> you probably meant:
>
>      data MList s = MList { ... , mlNextPh :: Mlist s ->  ... }
>
> Now I'm not sure about your exact problem since the following compiles
> for me.
>
>      import Data.Vector
>      import Data.Vector.Mutable
>      import Control.Monad.ST
>
>      type Move = ()
>      data MList s = MList {
>          mvVec :: MVector s Move,
>          mlNextPh :: MList s ->  ST s (Maybe (MList s)) }
>
>      splitMove :: MList s ->  ST s (Maybe (Move, MList s))
>      splitMove ml = do
>          m<- unsafeRead (mvVec ml) 0
>          undefined
>
> Something you always have to watch out for when dealing with ST is not
> to return something that depends on s in the last statement (the one
> you use runST on). In other words, if you want to return a vector you
> have to freeze it, so it's not mutable anymore.
>
> If you still can't figure it out paste some complete example that
> doesn't work.
>
> silvio
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