Intuition for MaybeT

Question

I'm trying to understand MaybeT by running a few examples in ghci:

λ: import Control.Monad.Trans.Maybe
λ: let x = return $ 42 :: MaybeT (Either String) Int
λ: :t x
x :: MaybeT (Either String) Int

Then, I ran it:

λ: runMaybeT x
Right (Just 42)

Please give me value, y, such that

runMaybeT y === Left (Just "...")
runMaybeT y === Left Nothing
runMaybeT y === Right Nothing

Answer 1

You will never get a Left Nothing or Left (Just ..), as the monad here is Either String …, so that in the left you will always have a String.

Here is what you can get:

> let y = fail "Failed" :: MaybeT (Either String) Int
> runMaybeT y
Right Nothing
> let y = lift (Left "Failed") :: MaybeT (Either String) Int
> runMaybeT y
Left "Failed"