Haskell中带可空列表参数的safeHead函数类型错误问题

你的实现代码：

safeHead :: Maybe [a] -> Maybe b
safeHead (Just []) = Nothing
safeHead Nothing = Nothing
safeHead (Just (x:xs)) = Just x

报错信息：

• Couldn't match expected type ‘b’ with actual type ‘a’
  ‘a’ is a rigid type variable bound by
    the type signature for:
      safeHead :: forall a b. Maybe [a] -> Maybe b
    at ch04.hs:2:1-32
  ‘b’ is a rigid type variable bound by
    the type signature for:
      safeHead :: forall a b. Maybe [a] -> Maybe b
    at ch04.hs:2:1-32
• In the first argument of ‘Just’, namely ‘x’
  In the expression: Just x
  In an equation for ‘safeHead’: safeHead (Just (x : xs)) = Just x
• Relevant bindings include
    xs :: [a] (bound at ch04.hs:4:19)
    x :: a (bound at ch04.hs:4:17)
    safeHead :: Maybe [a] -> Maybe b (bound at ch04.hs:3:1)

错误原因 #

问题出在类型签名的定义上：

• 你写的safeHead :: Maybe [a] -> Maybe b里，a和b是两个完全独立的多态类型变量，这意味着调用者可以随意指定返回值的类型b，比如传入Maybe [Int]却要求返回Maybe String。
• 但你最后一行代码返回的Just x中，x是输入列表的元素类型a，Haskell作为强类型语言，不允许这种无关联的类型强制匹配，类型系统无法将a自动转换成任意的b，因此抛出类型不匹配的错误。

修正方案 #

把类型签名中的返回类型改成Maybe a，让返回值的类型和输入列表的元素类型保持一致：

safeHead :: Maybe [a] -> Maybe a
safeHead (Just []) = Nothing
safeHead Nothing = Nothing
safeHead (Just (x:xs)) = Just x

此时Just x的类型是Maybe a，和函数的返回类型完全匹配，类型系统就会接受这个实现。

内容的提问来源于stack exchange，提问作者creatldd1