haskell中大小写匹配的替代功能

Question

I am writing an API in Haskell which involves a DB query. Function will be something like do a db query and then return the response back.

doAPICall :: Request -> (Monad m Response)
doAPICall req = do
    maybeValDB <- getValFromDB (req.tokenId) -- Returns maybe type
    -- My current logic was to do a case match of maybe
    case maybeValDB of
        Just val -> do
            --Some logic here using that val and return the response
            pure response
        Nothing -> pure response

I am currently getting a maybe type from the DB query function. Is there a simple way like a one liner code to avoid this case match and send the response back.

I thought of using fromMaybe but it should have same type for the default value. Here in this case if db returns Nothing I just want to exit the function with some response.

In general in many cases the response which I get will be of maybe type and I have to do case matches the code looks very ugly and have repetitive patterns. Can anyone tell me better alternative for this instead of having case matches everywhere.

Answer 1

Perhaps what you're looking for is maybe ?

doAPICall :: Request -> (Monad m Response)
doAPICall req = do
    maybeValDB <- getValFromDB (req.tokenId)
    flip (maybe $ pure response) maybeValDB $ \val -> do
      -- Some logic here using that val and return the response
      pure response