在Maybe中从（Just x）中提取x

Question

超级简单我很确定，但我似乎无法找到答案。我调用一个返回Maybe x的函数，我希望看到x。如何从x回复中提取Just 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视为：

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

Answer 2

您也可以使用fromMaybe，这是默认设置。

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

Answer 3

此签名有一个标准函数fromJust。

使用Hoogle进行此类搜索，it's a wonderful tool。