Question

我尝试使用undefined来获取类型常量的值（类似于sizeOf的值）。

module Main where

class MyClass a where
    typeConst :: a -> String
    -- ^ Argument is ignored

class TryRead a where
    tryRead :: String -> Maybe a

newtype ByLen a = ByLen a

-- | Make all types under ByLen readable, if their typeConst's are longer then 3 characters
instance (Read a, MyClass a) => TryRead (ByLen a) where
    tryRead = if len > 3
                then Just . ByLen . read
                else const Nothing
      where
        len = length $ typeConst (undefined :: a)

instance MyClass Int where
    typeConst = const "moreThan3"

main :: IO ()
main = go (tryRead "214234" :: Maybe (ByLen Int))
  where
    go Nothing = print "Nothing :("
    go (Just (ByLen i)) = print i

然而，这会产生错误：

Could not deduce (MyClass a0) arising from a use of ‘typeConst’
from the context (Read a, MyClass a)
  bound by the instance declaration at src/Main.hs:13:10-49
The type variable ‘a0’ is ambiguous
Note: there is a potential instance available:
  instance MyClass Int -- Defined at src/Main.hs:20:10
In the second argument of ‘($)’, namely
  ‘typeConst (undefined :: a)’
In the expression: length $ typeConst (undefined :: a)
In an equation for ‘len’: len = length $ typeConst (undefined :: a)

我不明白类型推断的问题是什么，考虑到我将typeConst参数的类型明确指定为a类型变量，它由MyClass a绑定，所以在我看来，应用typeConst函数没有问题。

引用实例声明中的类型

0 个答案: