Reader monad example

reader.hs

import Control.Monad.Reader

hello :: Reader String String
hello = do
    name <- ask
    return ("hello, " ++ name ++ "!")

bye :: Reader String String
bye = do
    name <- ask
    return ("bye, " ++ name ++ "!")

convo :: Reader String String
convo = do
    c1 <- hello
    c2 <- bye
    return $ c1 ++ c2

main = print . runReader convo $ "adit"

ok I see that shared data, but the readers are literally being unwrapped concatenated and then rewrapped... what about <$> and >>=?

answered my own question:

convo = hello >>= \h -> (bye >>= \b -> return $ h ++ b)

so the reader makes sure hello bye and the returned functions are all called with the same input

alternatively with fmap (<$>):

convo = hello >>= \h -> (\b -> h ++ b) <$> bye

or with apply (<*>)

import Control.Monad.Reader

hello :: Reader String String
hello = asks $ \name -> ("hello, " ++ name ++ "!")

bye :: Reader String String
bye = asks $ \name -> ("bye, " ++ name ++ "!")

convo :: Reader String String
convo = asks (const (++)) <*> hello <*> bye

main = print . runReader convo $ "adit"