Haskell “do nothing” IO, or if without else

Question

I want to do something in Haskell that looks like this:

main1 = do s <- getLine
           if s == \"foo\" then putStr \"You entered foo\"

Answer 1

The easiest way to do a no-op in a monad is:

return ()

Equivalently:

pure ()

However, for the particular idiom you're doing, there's a combinator already made for you:

import Control.Monad
main = do s <- getLine
          when (s == "foo") $ putStr "You entered foo"

This when combinator behaves exactly like your doIf combinator :)