Combination of readerT and maybeT

haskell-combine-readerT-MaybeT.hs

module CombineMonadT where

import Control.Monad.Trans
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Reader

getName :: MaybeT (ReaderT String IO) String
getName = MaybeT $ do
    input <- ask
    lift $ putStrLn input
    name <- lift $ getLine
    if name == ""
        then return Nothing
        else return $ Just name

getName' :: ReaderT String (MaybeT IO) String
getName' = ReaderT $
    \r -> MaybeT $ do
        putStrLn r
        name <- getLine
        if name == ""
            then return Nothing
            else return $ Just name

            
-- https://stackoverflow.com/questions/43840588/use-readert-maybe-or-maybet-reader

main :: IO ()
main = do
    mName <- runReaderT (runMaybeT getName) "type your name"
    case mName of
        Just name -> putStrLn $ "Hello " ++ name
        Nothing -> putStrLn "oh noooo!"

    nName' <- runMaybeT $ (runReaderT getName') "your name"
    case nName' of
        Just n -> putStrLn $ "Hello " ++ n
        Nothing -> putStrLn "oh noooo!"