Oh my god, that was it?
I looked at your code for half an hour, and I've never thought about that... That is really misleading.
So vector forces you to use strict ST? (That's right: http://hackage.haskell.org/packages/archive/primitive/0.4.1/doc/html/Control-Monad-Primitive.html#t:PrimMonad shows that only strict ST has a MonadPrime instance)

It's another plea against the "sames names/interface in different modules" pattern, that vector, ST, State, ByteString suffer from.
This (the error messages being misleading), as well as the document being hard to read (because you never know which type is mentionned, you have to see to check at the module from which the type comes. And even with that, it's just a matter of convention: for instance Control.Monad.State exports the lazy version, but Control.Monad.ST exports the strict one (so that the default version is closer to IO's behaviour).

I really prefer the approach taken by repa 3: one data family, generic functions for every flavour and some specific functions for each flavour. It's not perfect (not standard for instance), but I think such an approach should be priviledged in the future, it makes things much clearer, and enable you to choose between working generically (on 'Stuff a' types) or specifically (either only on 'Stuff Strict' or only on 'Stuff Lazy').
I would be interested to know if someone has other ideas in that respect in mind.


2012/6/9 Nicu Ionita <nicu.ionita@acons.at>
Ok, the error was: I was using Control.Monad.ST.Lazy. Importing Control.Monad.ST compiles immediately without problem. (Is this because I'm using unboxed mutable vectors?)

Now, that's a little bit odd.

It's clear that the strict and lazy forms of ST are different types. But unfortunately they are named the same! So actually any error message from the compiler drives you crazy, because it's refering to another type.

Probably the reason to name the types with the same name is for easy interchangeability. But as we see, the types are not (always) interchangeable.

Anyway, now it compiles.

Thanks,
Nicu

Am 08.06.2012 23:15, schrieb 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
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