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module Main where

Now, suppose that we want to accumulate output as we go. addline is a function which adds a line of text t to the string s.

addline ∷ String → String → String
addline s t = s ++ t ++ "\n"

Using addline, we can build our message in stages.

first, second, hello ∷ String

first = addline "" "hello"
second = addline first "there"
hello = addline second "world"

main ∷ IO ()
main = putStr hello

hello
there
world

Main.hs