在 Maybe [重复] 中从 (Just x) 中提取 x

Question

我敢肯定，超级简单，但我似乎找不到答案。我调用了一个返回Maybe x 的函数，我想查看x。如何从我的Just x 回复中提取x？

seeMyVal :: IO ()
seeMyVal = do
   if getVal == Nothing 
   then do
       putStrLn "Nothing to see here"
   else do
       putStrLn getVal -- what do I have to change in this line?

getVal :: (Maybe String)
getVal = Just "Yes, I'm real!"

这会引发错误：

Couldn't match type ‘Maybe String’ with ‘[Char]’
Expected type: String
  Actual type: Maybe String
In the first argument of ‘putStrLn’, namely ‘getVal’
In a stmt of a 'do' block: putStrLn getVal

Answer 1

惯用的方式是模式匹配。

seeMyVal = case getVal of
    Nothing  -> putStrLn "Nothing to see here"
    Just val -> putStrLn val

如果您愿意，可以将putStrLn 排除在外：

seeMyVal = putStrLn $ case getVal of
    Nothing  -> "Nothing to see here"
    Just val -> val

Answer 2

您也可以使用fromMaybe，它采用默认值。

fromMaybe "Nothing to see here..." (Just "I'm real!")

Answer 3

这个签名有一个标准函数fromJust。

使用 Hoogle 进行类似这样的搜索，it's a wonderful tool。