这里有一个简单的函数,可用于返回指针的对齐方式:
{-# LANGUAGE ScopedTypeVariables #-}
import Foreign.Ptr (Ptr)
import Foreign.Storable (Storable, alignment)
main = return ()
ptrAlign1 :: (Storable a) => Ptr a -> Int
ptrAlign1 _ = alignment (undefined :: a)
但是我收到了以下错误:
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
如果我以以下更凌乱的方式重新编写ptrAlign
:
ptrAlign2 :: (Storable a) => Ptr a -> Int
ptrAlign2 = ptrAlign3 undefined where
ptrAlign3 :: (Storable a) => a -> Ptr a -> Int
ptrAlign3 x _ = alignment x
它运行良好(当然,这个版本甚至不需要ScopedTypeVariables
)。
但我仍然很好奇为什么第一个版本会抛出错误,以及应该如何解决?