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module Main where

We could instead nest together all the calls to addline.

An empty string is an empty list of characters, so we can use generic list notation here, because the type can be deduced from context.

hello ∷ String
hello = addline (addline (addline [] first) second) third
   where
   addline s t = s ++ t ++ "\n"
   first = "hello"
   second = "there"
   third = "world"

main ∷ IO ()
main = putStr hello

hello
there
world

Main.hs