How does interact work with Haskell on a multiline string?

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I am trying to practise declarative programming using Haskell, I am running into some confusion with the interact function.

I can see from its type signature it operates on a whole string at once:

interact :: (String -> String) -> IO ()

What is the value of the string for a multiline input? I assume that it is a single string with a newline character inside of it?

For a Haskell program Main.hs

module Main where

main :: IO ()
main = interact( ... )

and a input file input.txt

5 6 7
3 6 10

If I run the compiled program like this:

$ Main < input.txt

Would the string that the interact is working with be:

5 6 7\n3 6 10
interactive functional-programming haskell
1 Answers

It will pass "5 6 7\n3 6 10\n" as string. We can easily verify that by using show as function:

main = interact show

If we then call the progam with I/O redirection, we get:

./Main < data.dat
"5 6 7\n3 6 10\n"

You can make use of lines :: String -> [String] to convert the String to a list of Strings where each string is a line. If we change the program to:

main = interact (show . lines)

we see:

./Main < data.dat
["5 6 7","3 6 10"]

So by using lines in this case, we retrieve a list of two elements "5 6 7", and "3 6 9".

user_675790
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