haskell – 讓重新命名功能中斷程式碼

在將程式碼迭代到正確的版本時,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

此程式碼不編譯：

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,如果我將iteratedF的定義更改為

iteratedF y = eval (iterateFunc f) y

程式碼編譯GHC 7.8.2.這個問題不是關於奇怪的簽名或資料型別,只是這樣：為什麼重新命名f到f’打破程式碼？這似乎是對我來說是個錯誤.