haskell – 让重命名功能中断代码

在将代码迭代到正确的版本时,我遇到了以下好奇心：

{-# LANGUAGE RankNTypes #-}

module Foo where

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

-- an in-place vector function with dimension
data DimFun v m r = 
  DimFun Int (v (PrimState m) r -> m ())

eval :: (PrimMonad m, MVector v r) => DimFun v m r -> v (PrimState m) r -> m ()
eval = error ""

iterateFunc :: (PrimMonad m, MVector v r)
            => (forall v' . (MVector v' r) => DimFun v' m r) -> DimFun v m r
iterateFunc = error ""

f :: (PrimMonad m, MVector v r)
      => DimFun v m r
f = error ""

iteratedF :: (MVector v r, PrimMonad m) 
           => v (PrimState m) r -> m ()
iteratedF y = 
    let f' = f
    in eval (iterateFunc f') y

此代码不编译：

Testing/Foo.hs:87:14:
    Could not deduce (MVector v0 r) arising from a use of ‘f’
    from the context (MVector v r, PrimMonad m)
      bound by the type signature for
                 iteratedF :: (MVector v r, PrimMonad m) =>
                              v (PrimState m) r -> m ()
      at Testing/Foo.hs:(84,14)-(85,39)
    The type variable ‘v0’ is ambiguous
    Relevant bindings include
      f' :: DimFun v0 m r (bound at Testing/Foo.hs:87:9)
      y :: v (PrimState m) r (bound at Testing/Foo.hs:86:11)
      iteratedF :: v (PrimState m) r -> m ()
        (bound at Testing/Foo.hs:86:1)
    In the expression: f
    In an equation for ‘f'’: f' = f
    In the expression: let f' = f in eval (iterateFunc f') y

Testing/Foo.hs:88:26:
    Couldn't match type ‘v0’ with ‘v'’
      because type variable ‘v'’ would escape its scope
    This (rigid, skolem) type variable is bound by
      a type expected by the context: MVector v' r => DimFun v' m r
      at Testing/Foo.hs:88:14-27
    Expected type: DimFun v' m r
      Actual type: DimFun v0 m r
    Relevant bindings include
      f' :: DimFun v0 m r (bound at Testing/Foo.hs:87:9)
    In the first argument of ‘iterateFunc’, namely ‘f'’
    In the first argument of ‘eval’, namely ‘(iterateFunc f')’
Failed, modules loaded: none.

但是,如果我将iteratedF的定义更改为

iteratedF y = eval (iterateFunc f) y

代码编译GHC 7.8.2.这个问题不是关于奇怪的签名或数据类型,只是这样：为什么重命名f到f’打破代码？这似乎是对我来说是个错误.