[Haskell-cafe] Functional dependency failure

[Alexander Pakhomov]

Hi!

I have following code:

STRead.hs:

import Control.Monad.Reader
import Control.Monad.ST
import qualified Data.Vector.Mutable as MV
import qualified Data.Vector as V

type ReadVecMonad s = ReaderT (MV.MVector s Int) (ST s)

freezeV :: ReadVecMonad s (V.Vector Int)
freezeV = ask >>= lift V.freeze

Trying to compile this I have "Couldn't match type ... When using functional dependencies to combine" error
following with kind mismatch and type with literaly the same type mismatch.

How can I work this around?

There's full error message:

STRead.hs:9:11:
    Couldn't match type `V.MVector s Int'
                  with `(->)
                          (V.MVector
                             (Control.Monad.Primitive.PrimState
                                (ReaderT (V.MVector s Int) (ST s)))
                             Int)'
    When using functional dependencies to combine
      MonadReader r (ReaderT r m),
        arising from the dependency `m -> r'
        in the instance declaration in `Control.Monad.Reader.Class'
      MonadReader
        ((->)
           (V.MVector
              (Control.Monad.Primitive.PrimState
                 (ReaderT (V.MVector s Int) (ST s)))
              Int))
        (ReaderT (V.MVector s Int) (ST s)),
        arising from a use of `ask' at STRead.hs:9:11-13
    In the first argument of `(>>=)', namely `ask'
    In the expression: ask >>= lift V.freeze


STRead.hs:9:19:
    Couldn't match kind `* -> *' with `*'
    Expected type: (->)
                     (V.MVector
                        (Control.Monad.Primitive.PrimState
                           (ReaderT (V.MVector s Int) (ST s)))
                        Int)
                   -> ReaderT (V.MVector s Int) (ST s) (V.Vector Int)
      Actual type: (->)
                     (V.MVector
                        (Control.Monad.Primitive.PrimState
                           (ReaderT (V.MVector s Int) (ST s)))
                        Int)
                   -> ReaderT (V.MVector s Int) (ST s) (V.Vector Int)
    Kind incompatibility when matching types:
      (->)
        (V.MVector
           (Control.Monad.Primitive.PrimState
              (ReaderT (V.MVector s Int) (ST s)))
           Int) :: * -> *
      (->)
        (V.MVector
           (Control.Monad.Primitive.PrimState
              (ReaderT (V.MVector s Int) (ST s)))
           Int) :: *
    In the return type of a call of `lift'
    In the second argument of `(>>=)', namely `lift V.freeze'

STRead.hs:9:24:
    Couldn't match kind `*' with `* -> *'
    Expected type: V.MVector
                     (Control.Monad.Primitive.PrimState
                        (ReaderT (V.MVector s Int) (ST s)))
                     Int
                   -> ReaderT (V.MVector s Int) (ST s) (V.Vector Int)
      Actual type: V.MVector
                     (Control.Monad.Primitive.PrimState
                        (ReaderT (V.MVector s Int) (ST s)))
                     Int
                   -> ReaderT (V.MVector s Int) (ST s) (V.Vector Int)
    Kind incompatibility when matching types:
      (->)
        (V.MVector
           (Control.Monad.Primitive.PrimState
              (ReaderT (V.MVector s Int) (ST s)))
           Int) :: *
      (->)
        (V.MVector
           (Control.Monad.Primitive.PrimState
              (ReaderT (V.MVector s Int) (ST s)))
           Int) :: * -> *
    In the first argument of `lift', namely `V.freeze'
    In the second argument of `(>>=)', namely `lift V.freeze'