Haskell - loop over user input

Question

I have a program in haskell that has to read arbitrary lines of input from the user and when the user is finished the accumulated input has to be sent to a function.

In

Answer 1

It's reasonably simple in Haskell. The trickiest part is that you want to accumulate the sequence of user inputs. In an imperative language you use a loop to do this, whereas in Haskell the canonical way is to use a recursive helper function. It would look something like this:

getUserLines :: IO String                      -- optional type signature
getUserLines = go ""
  where go contents = do
    line <- getLine
    if line == "q"
        then return contents
        else go (contents ++ line ++ "\n")     -- add a newline

This is actually a definition of an IO action which returns a String. Since it is an IO action, you access the returned string using the <- syntax rather than the = assignment syntax. If you want a quick overview, I recommend reading The IO Monad For People Who Simply Don't Care.

You can use this function at the GHCI prompt like this

>>> str <- getUserLines
Hello     -- user input
World     -- user input
q         -- user input
>>> putStrLn str
Hello            -- program output
World            -- program output