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
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