Question

我试图通过学习你是一个很好的Haskell来完成应用实例！书。

我的代码在这里：

class (Functor f) => Applicative f where  
    pure :: a -> f a  
    (<*>) :: f (a -> b) -> f a -> f b

instance Applicative Maybe where  
    pure = Just  
    Nothing <*> _ = Nothing  
    (Just f) <*> something = fmap f something  

main = do 
    print $ Just (+3) <*> Just 9 
    print $ pure (+3) <*> Just 10
    print $ pure (+3) <*> Just 9 
    print $ Just (++"hahah") <*> Nothing  
    print $ Nothing <*> Just "woot"

我收到了此错误消息：

No instance for (Show b0) arising from a use of `print'
The type variable `b0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance Show Double -- Defined in `GHC.Float'
  instance Show Float -- Defined in `GHC.Float'
  instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
    -- Defined in `GHC.Real'
  ...plus 23 others
In the expression: print
In a stmt of a 'do' block: print $ Nothing <*> Just "woot"
In the expression:
  do { print $ Just (+ 3) <*> Just 9;
       print $ pure (+ 3) <*> Just 10;
       print $ pure (+ 3) <*> Just 9;
       print $ Just (++ "hahah") <*> Nothing;
       .... }

为什么会失败，我该如何解决？

Answer 1

问题出在最后print行。基本上，您使用String -> b应用函数<*>。只是碰巧你使用Nothing来包装函数，所以最后没有函数。该应用程序的结果是Maybe b b，其中a是任意的，但只有那些Show Maybe a的实例为b生成实例。但请记住，您有一个Show，它不一定是print的实例。但是Show要求其参数是b的实例。由于您无法证明Show是Maybe b的实例，因此您无法证明Show是print的实例，因此Maybe b不适用于该值类型为(Nothing :: Maybe (String -> Int)) <*> ...。

要使它消失，您必须指定您想要包含的确切类型的函数。所以结果有一些已知的类型：

print

或者你可以只提供作为print (... :: Maybe Int)的输入传递的整个表达式的类型：

   UPDATE your_tablename 
   SET SetType = 'CATEGORY_'+ CONVERT(NVarchar(2),SetType)

没有（Show b0）使用'print＆＃39;类型变量`b0＆＃39;很暧昧

1 个答案: