Haskell异构可存储对象列表

我想写一个函数，它可以创建一个可存储对象（不同类型）的异构列表

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']

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？

简言之，否。您需要的是

存在

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

可以有效地从另一端表达相同的内容

---

BTW，您可以考虑制作<代码>可存储的< /C> >代码>可存储的< /代码> .< /P>当然，您是对的。code>AnyStorable必须是

Storable

的实例。

{-# 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']