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module Main where

By using a where clause, we can make all of the needed definitions local to hello.

Note the use of indentation. Precise rules exist, and one could use braces instead. In practice one learns indentation styles from other people’s code, and through trial and error seeing what the compiler will accept.

concatMap is a combination of concat and map.

concatMap ∷ (a → [b]) → [a] → [b]

hello ∷ String
hello = concatMap endline [first, second, third]
   where
   endline = (++ "\n")
   first = "hello"
   second = "there"
   third = "world"

main ∷ IO ()
main = putStr hello

hello
there
world

Main.hs