Estou tentando entender por que uma versão desse código é compilada e outra não.

{-# LANGUAGE RankNTypes, FlexibleContexts #-}

module Foo where

import Data.Vector.Generic.Mutable as M
import Data.Vector.Generic as V
import Control.Monad.ST
import Control.Monad.Primitive

data DimFun v m r = 
  DimFun {dim::Int, func :: v (PrimState m) r -> m ()}

runFun1 :: (Vector v r, MVector (Mutable v) r) => 
  (forall m . (PrimMonad m) => DimFun (Mutable v) m r) -> v r -> v r
runFun1 (DimFun dim t) x | V.length x == dim = runST $ do
  y <- thaw x
  t y
  unsafeFreeze y

runFun2 :: (Vector v r, MVector (Mutable v) r) => 
  (forall m . (PrimMonad m) => DimFun (Mutable v) m r) -> v r -> v r
runFun2 t x = runST $ do
  y <- thaw x
  evalFun t y
  unsafeFreeze y

evalFun :: (PrimMonad m, MVector v r) => DimFun v m r -> v (PrimState m) r -> m ()
evalFun (DimFun dim f) y | dim == M.length y = f y

runFun2 compila bem (GHC-7.8.2), masrunFun1 resulta em erros:

Could not deduce (PrimMonad m0) arising from a pattern
from the context (Vector v r, MVector (Mutable v) r)
  bound by the type signature for
             tfb :: (Vector v r, MVector (Mutable v) r) =>
                    (forall (m :: * -> *). PrimMonad m => TensorFunc m r) -> v r -> v r
  at Testing/Foo.hs:(26,8)-(28,15)
The type variable ‘m0’ is ambiguous
Note: there are several potential instances:
  instance PrimMonad IO -- Defined in ‘Control.Monad.Primitive’
  instance PrimMonad (ST s) -- Defined in ‘Control.Monad.Primitive’
In the pattern: TensorFunc _ f
In an equation for ‘tfb’:
    tfb (TensorFunc _ f) x
      = runST
        $ do { y <- thaw x;
               f y;
               unsafeFreeze y }

Couldn't match type ‘m0’ with ‘ST s’
  because type variable ‘s’ would escape its scope
This (rigid, skolem) type variable is bound by
  a type expected by the context: ST s (v r)
  at Testing/Foo.hs:(29,26)-(32,18)
Expected type: ST s ()
  Actual type: m0 ()
Relevant bindings include
  y :: Mutable v s r (bound at Testing/Foo.hs:30:3)
  f :: forall (v :: * -> * -> *).
       MVector v r =>
       v (PrimState m0) r -> m0 ()
    (bound at Testing/Foo.hs:29:19)
In a stmt of a 'do' block: f y
In the second argument of ‘($)’, namely
  ‘do { y <- thaw x;
        f y;
        unsafeFreeze y }’

Could not deduce (s ~ PrimState m0)
from the context (Vector v r, MVector (Mutable v) r)
  bound by the type signature for
             tfb :: (Vector v r, MVector (Mutable v) r) =>
                    (forall (m :: * -> *). PrimMonad m => TensorFunc m r) -> v r -> v r
  at Testing/Foo.hs:(26,8)-(28,15)
  ‘s’ is a rigid type variable bound by
      a type expected by the context: ST s (v r) at Testing/Foo.hs:29:26
Expected type: Mutable v (PrimState m0) r
  Actual type: Mutable v s r
Relevant bindings include
  y :: Mutable v s r (bound at Testing/Foo.hs:30:3)
  f :: forall (v :: * -> * -> *).
       MVector v r =>
       v (PrimState m0) r -> m0 ()
    (bound at Testing/Foo.hs:29:19)
In the first argument of ‘f’, namely ‘y’
In a stmt of a 'do' block: f y

Tenho certeza de que o tipo rank-2 é o culpado, possivelmente causado por uma restrição de monomorfismo. No entanto, como sugerido em umpergunta anterior meu, eu habilitei-XNoMonomorphismRestriction, mas obteve o mesmo erro.

Qual é a diferença entre esses trechos de código aparentemente idênticos?