applying functor on print

Question

I've been trying to print 2 values separately, and I tried this code:

import System.Directory
main = getCurrentDirectory >>= \x -> (print <$> doesFileExist x) >> (print <$> doesDirectoryExist x)

but it doesn't print anything however the following code works fine:

import System.Directory
main = getCurrentDirectory >>= \x -> doesFileExist x >>= print >> doesDirectoryExist x >>= print

any reasons for why the 1st code doesn't print anything ?

Answer 1

If you fmap print over an IO action, you don't get an IO action that performs this printing. You just get an IO action that performs whatever side-effects the original action had, but instead of yielding the printable value as the result it yields another IO action as the result which you could then execute in a separate step:

import Control.Applicative
import Data.Time

printCurrentTime :: IO ()
printCurrentTime = do
   tPrinter <- print <$> getCurrentTime
   tPrinter

or, without do notation,

printCurrentTime = print <$> getCurrentTime >>= \tPrinter -> tPrinter

In other words,

printCurrentTime = print <$> getCurrentTime >>= id

By the monad laws, f <$> a >>= b is the same as a >>= b . f, i.e.

printCurrentTime = getCurrentTime >>= id . print

which is the same as simply

printCurrentTime = getCurrentTime >>= print

That could than be written with do notation as

printCurrentTime = do
   t <- getCurrentTime
   print t