问题
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
回答1:
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.
来源：https://stackoverflow.com/questions/19288652/difference-between-print-and-putstrln-in-haskell