haskell - 简化 IO block 中的 Maybe monad

Question

假设我有一个函数:

get :: String -> Maybe [(C8.ByteString, Maybe C8.ByteString)] -> IO (Response LC8.ByteString)
get url qParams' = do
  request <- parseUrl url

  --todo simplify
  let qParams = case qParams' of
                  Just p -> p
                  Nothing -> [] 

  res <- withManager $ httpLbs $ configRequest request qParams
  return res

我相信有一种方法可以简化 let qParams = ...表达。但我不知道。我本来可以用 Just qParams <- qParams'如果它是 Maybe monad，但它是 IO monad，所以我不能使用它。

你的想法？

Answer 1

使用maybe功能:

let qParams = maybe [] id qParams'

或者正如@MichaelSnoyman建议的那样，您可以使用 fromMaybe :

let qParams = fromMaybe [] qParams'