Re: [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:
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/
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=X9CL
-----END PGP SIGNATURE-----


_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe