haskell - ScopedTypeVariables 不会将类型变量带入作用域

Question

这是一个返回指针对齐方式的简单函数:

{-# LANGUAGE ScopedTypeVariables #-}

import Foreign.Ptr (Ptr)
import Foreign.Storable (Storable, alignment)

main = return ()

ptrAlign1 :: (Storable a) => Ptr a -> Int
ptrAlign1 _ = alignment (undefined :: a) 

但我收到以下错误:

Could not deduce (Storable a0) arising from a use of `alignment'
from the context (Storable a)
  bound by the type signature for
             ptrAlign1 :: Storable a => Ptr a -> Int
  at prog.hs:8:14-41
The type variable `a0' is ambiguous

如果我像这样以更困惑的方式重写 ptrAlign :

ptrAlign2 :: (Storable a) => Ptr a -> Int
ptrAlign2 = ptrAlign3 undefined where
  ptrAlign3 :: (Storable a) => a -> Ptr a -> Int
  ptrAlign3 x _ = alignment x

它工作得很好(当然这个版本甚至不需要ScopedTypeVariables)。

但我仍然好奇为什么第一个版本会抛出错误，以及可以采取什么措施来解决它？

Answer 1

即使打开了 ScopedTypeVariables，类型变量也不会放入作用域中，除非您显式量化它们，即

ptrAlign1 :: forall a. (Storable a) => Ptr a -> Int
ptrAlign1 _ = alignment (undefined :: a)