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module Main where

hello ∷ String
hello = "hello"

The definition of endline can be simplified by using an operator section. Leaving s out of the parenthesized concatenation creates an unnamed function with the same type as endline.

endline ∷ String → String
endline = (++ "\n")

The $ operator applies its left argument to its right argument. Here, we use $ in place of parentheses.

main ∷ IO ()
main = putStr $ endline hello

hello

Main.hs