如何从Maybe中提取IO（）并执行此操作？(How can I extract an IO () from Maybe and execute this?)
我有这个代码：
trick = Just (putStrLn "Hello?")我想将这个
IO ()从Maybe上下文解包并调用它。main = do foo <- trick foo但是，这会引发错误：
Couldn't match type ‘IO’ with ‘Maybe’ Expected type: Maybe () Actual type: IO ()我怎么解决这个问题？
I have this code:
trick = Just (putStrLn "Hello?")And I want to unwrap this
IO ()out the Maybe context and call it.main = do foo <- trick fooHowever, this throws an error:
Couldn't match type ‘IO’ with ‘Maybe’ Expected type: Maybe () Actual type: IO ()How could I resolve this?
原文：https://stackoverflow.com/questions/29516861
更新时间：2023-03-10 18:03
最满意答案
如果您阅读了count函数的文档 ，您将在本节中找到有关返回值的信息：
返回值
返回array_or_countable中的元素数。 如果参数不是数组或不是具有已实现Countable接口的对象， 则将返回1 。 有一个例外，如果array_or_countable为NULL，则返回0。
If you read the documentation of the count function, you'll find this section about the return value:
Return Values
Returns the number of elements in array_or_countable. If the parameter is not an array or not an object with implemented Countable interface, 1 will be returned. There is one exception, if array_or_countable is NULL, 0 will be returned.
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