Question

我还在学习Haskell，需要帮助进行类型推断！

使用包SDL和Yampa 我从FRP.Yampa.reactimate获得以下类型签名：

(Bool -> IO (DTime, Maybe a))

我希望将其用于：

myInput :: Bool -> IO (DTime, Maybe [SDL.Event])
myInput isBlocking = do
    event <- SDL.pollEvent
    return (1, Just [event])
...
reactimate myInit myInput myOutput mySF

但它说

Couldn't match expected type `()'
       against inferred type `[SDL.Event]'
  Expected type: IO (DTime, Maybe ())
  Inferred type: IO (DTime, Maybe [SDL.Event])
In the second argument of `reactimate', namely `input'
In the expression: reactimate initialize input output process

我认为Maybe a允许我使用任何内容，甚至是SDL.Event列表？当类型签名实际为Maybe ()时，为什么期望Maybe a？为什么它需要一个空元组，或一个不带参数的函数，或者()应该是什么？

Answer 1

完整的type signature of reactimate是

IO a                               -- # myInit
 -> (Bool -> IO (DTime, Maybe a))  -- # myInput
 -> (Bool -> b -> IO Bool)         -- # myOutput
 -> SF a b                         -- # mySF
 -> IO ()

相同的a和b必须匹配，这意味着如果您的myInput类型为Bool -> IO (DTime, Maybe [SDL.Event])，则所有其他a也必须为{{1} }}。因此，要匹配类型，您需要确保

[SDL.Event]

BTW，myInit :: IO [SDL.Event] -- # **not** IO (). mySF :: SF [SDL.Event] b是unit type。

类型签名“也许”不喜欢“Just [Event]”

1 个答案: