haskell - 让重命名函数破坏代码

Question

在将代码迭代到正确版本时，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

此代码无法编译:

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，如果我更改 iteratedF 的定义至

iteratedF y = eval (iterateFunc f) y

该代码使用 GHC 7.8.2 编译。这个问题与看起来很奇怪的签名或数据类型无关，它只是这样:为什么重命名 f至 f'破解密码？这对我来说似乎是一个错误。

Answer 1

禁用单态限制，我可以编译你的代码。所以，只需添加

{-# LANGUAGE NoMonomorphismRestriction #-}

在文件的开头。

类型错误的原因是定义

let f' = f

不使用函数模式(例如 f' x y = ... )，因此单态限制开始生效并强制 f'是单态的，而 iterateFunc需要一个多态函数。

或者，添加类型注释

let f' :: (PrimMonad m, MVector v r) => DimFun v m r
    f' = f