haskell - 在Maybe中从(Just x)中提取x

Question

这个问题在这里已经有了答案:





Operating on a return from a Maybe that contains "Just"

(2 个回答)


5年前关闭。




我敢肯定， super 简单，但我似乎找不到答案。我调用了一个返回 Maybe x 的函数我想看 x .如何提取 x来自我的 Just x回复？

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

惯用的方法是模式匹配。

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

如果您愿意，可以考虑 putStrLn出去:

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