如何在绑定操作中使用非monadic函数

Question

I feel like I should already know this, but how could I use fromMaybe in one line instead of breaking it into 2 with a let ?

main = do
    maybePort <- lookupEnv "PORT"
    let port = fromMaybe "4020" maybePort
    putStrLn $ "Listening on:" ++ port

Answer 1

you can use fmap or <$> like this:

import Control.Applicative ((<$>))

main = do
    port <- fromMaybe "4020" <$> lookupEnv "PORT"
    putStrLn $ "Listening on:" ++ port