Difference between print and putStrLn in Haskell

haskell printing ghc
Amir Nabaei picture Amir Nabaei · Oct 10, 2013 · Viewed 29.5k times · Source

I am confused. I tried to use print, but I know people apply putStrLn. What are the real differences between them?

print $ function 
putStrLn $ function

Answer

Chris Taylor picture Chris Taylor · Oct 10, 2013

The function putStrLn takes a String and displays it to the screen, followed by a newline character (put a String followed by a new Line).

Because it only works with Strings, a common idiom is to take any object, convert it to a String, and then apply putStrLn to display it. The generic way to convert an object to a String is with the show function, so your code would end up with a lot of

putStrLn (show 1)
putStrLn (show [1, 2, 3])
putStrLn (show (Just 42))

Once you notice that, it's not a very big stretch to define a function that converts to a String and displays the string in one step

print x = putStrLn (show x)

which is exactly what the print function is.

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