haskell - 在 Data.Array.Unboxed 中使用 newtype 和 ghc 7.10

Question

这在 ghc 7.8.4 中工作正常，但在 7.10.3 中失败:

{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Foo where

import qualified Data.Array.Unboxed       as A
import           GHC.Generics             (Generic)

newtype Elt = Elt Int deriving (Eq, Ord, Show, Num, Integral, Real, Enum, A.IArray A.UArray, Generic)
type MyArr = A.UArray Int Elt

有很多消息，例如

/tmp/my.hs:9:75:
    Couldn't match type ‘Int’ with ‘Elt’
    arising from the coercion of the method ‘Data.Array.Base.numElements’
      from type ‘forall i. A.Ix i => A.UArray i Int -> Int’
        to type ‘forall i. A.Ix i => A.UArray i Elt -> Int’
    Relevant role signatures:
      type role A.Ix nominal
      type role A.UArray nominal nominal
    When deriving the instance for (A.IArray A.UArray Elt)

虽然 7.10 的发行说明没有提到这一点，但我明白了 https://ghc.haskell.org/trac/ghc/ticket/9220#comment:11承认这是一个突破性的改变。但解决方案是什么 - 我真的必须为 MyArr 创建一个新类型包装器，并为每种用途提供辅助函数吗？

Answer 1

您不必为 MyArr 创建包装器，但您必须手动写出您之前派生的实例。一种强力解决方案是手动通过 IArray 实例进行 unsafeCoerce 操作(无法coerce 的原因与您相同)无法导出)。

{-# LANGUAGE InstanceSigs, ScopedTypeVariables, MultiParamTypeClasses #-}

import Data.Array.Base
import Data.Array.IArray
import Data.Array.Unboxed
import Unsafe.Coerce

instance IArray UArray Elt where
  bounds :: forall i. Ix i => UArray i Elt -> (i, i)
  bounds arr = bounds (unsafeCoerce arr :: UArray i Int)                                                                                                                                                                        

  numElements :: forall i. Ix i => UArray i Elt -> Int
  numElements arr = numElements (unsafeCoerce arr :: UArray i Int)

  unsafeArray :: forall i. Ix i => (i,i) -> [(Int, Elt)] -> UArray i Elt
  unsafeArray lu ies = unsafeCoerce (unsafeArray lu [ (i,e) | (i,Elt e) <- ies ] :: UArray i Int) :: UArray i Elt

  unsafeAt :: forall i. Ix i => UArray i Elt -> Int -> Elt
  unsafeAt arr ix = Elt (unsafeAt (unsafeCoerce arr :: UArray i Int) ix)