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我想编写可以戳可存储对象（不同类型）的异构列表的函数

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE RankNTypes, ExistentialQuantification, ImpredicativeTypes #-}

pokeMany :: Ptr b -> Int -> [forall a. Storable a => a] -> IO ()
pokeMany _ _ [] = return ()
pokeMany ptr offset (x:xs) = do
    pokeByteOff ptr offset x
    pokeMany ptr (offset + sizeOf x) xs

somePoke :: Ptr a -> Int -> Int -> IO ()
somePoke ptr x1 x2 = pokeMany ptr 0 [x1, x2]

但我得到编译错误：

No instance for (Storable a1) arising from a use of `sizeOf'
The type variable `a1' is ambiguous
Note: there are several potential instances:
  instance Storable CChar -- Defined in `Foreign.C.Types'
  instance Storable CClock -- Defined in `Foreign.C.Types'
  instance Storable CDouble -- Defined in `Foreign.C.Types'

Couldn't match expected type `a2' with actual type `Int'
  `a2' is a rigid type variable bound by
       a type expected by the context: Storable a2 => a2

和别的 ...

然后我创建一些数据类型

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE RankNTypes, ExistentialQuantification, ImpredicativeTypes #-}

data AnyStorable = forall a. Storable a => AnyStorable a

pokeMany :: Ptr b -> Int -> [AnyStorable] -> IO ()
pokeMany _ _ [] = return ()
pokeMany ptr offset (AnyStorable x:xs) = do
    pokeByteOff ptr offset x
    pokeMany ptr (offset + sizeOf x) xs

somePoke :: Ptr a -> Int -> Int -> IO ()
somePoke ptr n c = pokeMany ptr 0 [AnyStorable n, AnyStorable c, AnyStorable 'a']

上面的代码没有任何编译错误。

我可以编写pokeMany函数而不创建像 AnyStorable 这样的新数据类型吗？

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1 回答 1

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简而言之，没有。您需要的是exists关键字来指定将映射到集合上的函数的签名。在数据类型中使用forall有效地表达了另一面的相同。

AnyStorable顺便说一句，您可以考虑制作Storable.

于 2016-07-08T07:52:02.100 回答