Hi, After trying the whole afternoon to make a program work using ST and mutable vectors, I must give up and ask for some help. I have a pure function which generates a list of moves. But the whole thing should live in the ST monad, so:
genMoves ... = runST $ do ...
Now, as I understand, I have a private universe (under runST) in which I can run "impure code", from which nothing escapes to the outside. Now in that universe I prepare succesively (and use later) a data structure which contains pure and impure values, for example:
data MList = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe MList) }
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:
data MList s = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe (MList s)) }
but then, when I define functions like:
splitMove :: MList s -> ST s (Maybe (Move, MList s)) splitMove ml = do m <- unsafeRead (mvVec ml) 0 ...
I get this message: Moves\MoveList.hs:217:28: Couldn't match type `s' with `PrimState (ST s)' `s' is a rigid type variable bound by the type signature for splitMove :: MList s -> ST s (Maybe (Move, MList s)) at Moves\MoveList.hs:210:1 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 first argument of `M.unsafeRead', namely `(mlVec ml)' which really doesn't make sense, as the package primitive defines the instance: instance PrimMonad (ST s) where type PrimState (ST s) = s primitive = ST internal (ST p) = p Should I do the structure agnostic of that s-state? (forall s. ...) This seems really unintuitive to me... Anybody some hint? Thanks, Nicu