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module Main where

We can replace the repeated calls to addline with a single operation, by using a fold. One can fold from either the left or the right, combining an initial value of type a with a list of values of type b, to produce a result of type a. The function foldl folds from the left; foldr folds from the right.

foldl ∷ (a → b → a) → a → [b] → a
foldr ∷ (b → a → a) → a → [b] → a

We are combining from the left, so we use foldl.

hello ∷ String
hello = foldl 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