[Haskell-cafe] Vector mysteries

[Fixie Fixie]

Hi

I am trying to implement Show for a storable-mutable-vector.

Does anyone have a clue why the type-system is killing me :-)

Code is below...

Cheers

Felix

----

{-# LANGUAGE ExistentialQuantification #-}

import Control.Monad (liftM2)
import qualified Data.Vector.Unboxed as V
import qualified Data.Vector.Unboxed.Mutable as MV
import Data.Vector.Storable.Mutable
import GHC.Prim (RealWorld)
import Control.Monad.Primitive
import Control.Monad
import qualified Data.Vector as VEC
import qualified Data.Vector.Generic.Mutable as GM
import Data.Int
import Data.Typeable.Internal
import Data.Primitive

data Row = forall m s. (Storable s, MV.Unbox s, Prim s, PrimMonad m) => Row (MV.MVector (PrimState m) s)

instance Show Row where
    show (Row row) = do
                    let xx = MV.read row 0
                    "Done"

main :: IO ()
main = do
    print "Done"

I get this error:

    Could not deduce (PrimState m ~ PrimState m0)
    from the context (Storable s, MV.Unbox s, Prim s, PrimMonad m)
      bound by a pattern with constructor
                 Row :: forall (m :: * -> *) s.
                        (Storable s, MV.Unbox s, Prim s, PrimMonad m) =>
                        MV.MVector (PrimState m) s -> Row,
               in an equation for `show'
    NB: `PrimState' is a type function, and may not be injective
    Expected type: MV.MVector (PrimState m0) s
      Actual type: MV.MVector (PrimState m) s
    In the first argument of `MV.read', namely `row'
    In the expression: MV.read row 0
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