haskell - Haskell:类和函数重载:

我对Haskell很陌生。我正在尝试为不同的容器重载自定义函数“ myadd”。我的理解是，您必须通过课堂来做到这一点。这是我尝试的：

class Addition a where
  myadd :: a -> a -> a

instance Addition Int where
  myadd a b = a + b

instance Addition Maybe where
  myadd (Just a) (Just b) = Just (a + b)

main = do
  let a = 3 :: Int
  let b = 4
  let c = myadd a b
  print c

  let d = (Just a)
  let e = (Just b)
  let f = myadd d e
  print f

但我得到以下错误：

test.hs:7:19: error:
    ? Expecting one more argument to ‘Maybe’
      Expected a type, but ‘Maybe’ has kind ‘* -> *’
    ? In the first argument of ‘Addition’, namely ‘Maybe’
      In the instance declaration for ‘Addition Maybe’

最佳答案

Addition具有类型* -> *，因此Addition的参数必须为类型*。 Maybe具有种类* -> *。您需要将其他类型参数传递给Maybe，以使其具有类型*。

instance Addition (Maybe a)

问题在于，对于任意Maybe a，Addition都不是a的实例，而对于已经是a实例的Addition而言，它仅是a的实例。您可以这样指定Addition是myadd的实例的要求：

instance Addition a => Addition (Maybe a)

同样，您的实现应使用+而不是：

instance Addition a => Addition (Maybe a) where
   myadd (Just a) (Just b) = Just (myadd a b)