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