簡單資料型別的未裝箱向量

我想在一個簡單的新型別上使用未裝箱的向量，但我不清楚如何啟用它。到目前為止我所擁有的：

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}

module UnboxedTest where

import Data.Vector.Unboxed
import Data.Word

newtype X = X Word64 deriving (Unbox)

我得到：

src/UnboxedTest.hs:11:32: error:
    ? No instance for (Data.Vector.Generic.Base.Vector Vector X)
        arising from the 'deriving' clause of a data type declaration
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    ? When deriving the instance for (Unbox X)
   |
11 | newtype X = X Word64 deriving (Unbox)

如何將其放入未裝箱的向量中？

uj5u.com熱心網友回復：

如果有一個同構型別已經可以存盤在未裝箱的向量中，那就很簡單了。因為Point那是(Word64, Word64)。

您必須按照該檔案頁面的說明進行操作并撰寫：

{-# LANGUAGE TypeFamilies #-}
import Data.Vector.Unboxed
import Data.Vector.Unboxed.Mutable (MVector)
import qualified Data.Vector.Generic as G
import qualified Data.Vector.Generic.Mutable as GM
import Data.Word

data Point = Point Word64 Word64

newtype instance MVector s Point = MV_Point (MVector s (Word64,Word64))
newtype instance Vector    Point = V_Point  (Vector    (Word64,Word64))

instance GM.MVector MVector Point where
  {-# INLINE basicLength #-}
  basicLength (MV_Point v) = GM.basicLength v
  {-# INLINE basicUnsafeWrite #-}
  basicUnsafeWrite (MV_Point v) i (Point x y) = GM.basicUnsafeWrite v i (x, y)
  ...

instance G.Vector Vector Point where
  {-# INLINE basicLength #-}
  basicLength (V_Point v) = G.basicLength v
  {-# INLINE basicUnsafeIndexM #-}
  basicUnsafeIndexM (V_Point v) i = do
    (x, y) <- G.basicUnsafeIndexM v i
    pure (Point x y)
  ...

instance Unbox Point

這些宣告指定如何將您的Point轉換為 a(Word64, Word64)以存盤在未裝箱的向量中。更明顯的是，如果您查看例如basicUnsafeWriteandbasicUnsafeIndexM函式，您可以看到實際上確實需要撰寫一些代碼來轉換Point和(Word64, Word64)回傳，這些并不是完全多余的。

uj5u.com熱心網友回復：

盡管 Haskell 的各種派生機制仍然無法生成與型別族相關的類的實體（就像Unboxed.Vector在本例中一樣），但始終有可能使用 Template Haskell 生成實體，它基本上可以做任何你可以手動做的事情。幸運的是，有人已經在包Unbox中撰寫了生成實體的宏：vector-th-unbox

{-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, TypeFamilies, FlexibleInstances #-}

import Data.Vector.Unboxed.Deriving (derivingUnbox)

newtype X = X Word64

derivingUnbox "X"
   [t| X -> Word64 |]
   [| \(X w) -> w |]
   [| X |]

實際上，由于這是一種新型別，我很想建議使用更簡單的宏：

import Data.Coerce
import Language.Haskell.TH

derivingNewtypeUnbox :: String -> TypeQ -> DecsQ
derivingNewtypeUnbox tfq tg = derivingUnbox tfq tg [|coerce|] [|coerce|]

然后它只是

derivingNewtypeUnbox "X" [t| X -> Word64 |]

uj5u.com熱心網友回復：

由于版本vector-0.13可以用于GeneralizedNewtypeDeriving派生Unbox，請參閱檔案

這是黑線鱈的例子：

>>> :set -XTypeFamilies -XStandaloneDeriving -XMultiParamTypeClasses -XGeneralizedNewtypeDeriving
>>> 
>>> import qualified Data.Vector.Unboxed         as U
>>> import qualified Data.Vector.Generic         as G
>>> import qualified Data.Vector.Generic.Mutable as M
>>> 
>>> newtype Foo = Foo Int
>>> 
>>> newtype instance U.MVector s Foo = MV_Int (U.MVector s Int)
>>> newtype instance U.Vector    Foo = V_Int  (U.Vector    Int)
>>> deriving instance M.MVector MVector Foo
>>> deriving instance G.Vector  Vector  Foo
>>> instance Unbox Foo

標籤：哈斯克尔向量拆箱

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