Haskell 可存储对象的异构列表

Question

我想编写一个函数，可以找出可存储对象的异构列表（不同类型）

{-# 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]

但是我遇到编译错误：

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

和其他人...

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']

以上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？

Answer 1

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 从另一面有效地表达了相同的意思。

顺便说一句，您可以考虑将 AnyStorable 作为 Storable.

的实例

Haskell 可存储对象的异构列表

Haskell heterogeneous list of Storable objects

haskell

polymorphism

existential-type

heterogeneous