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:
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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.
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