Is print in Haskell a pure function?

Question

Is print in Haskell a pure function; why or why not? I'm thinking it's not, because it does not always return the same value as pure functions should.

Answer 1

A value of type IO Int is not really an Int. It's more like a piece of paper which reads "hey Haskell runtime, please produce an Int value in such and such way". The piece of paper is inert and remains the same, even if the Ints eventually produced by the runtime are different.

You send the piece of paper to the runtime by assigning it to main. If the IO action never comes in the way of main and instead languishes inside some container, it will never get executed.

Functions that return IO actions are pure like the others. They always return the same piece of paper. What the runtime does with those instructions is another matter.

If they weren't pure, we would have to think twice before changing

foo :: (Int -> IO Int) -> IO Int
foo f = liftA2 (+) (f 0) (f 0)

to:

foo :: (Int -> IO Int) -> IO Int
foo f = let x = f 0 in liftA2 (+) x x

Answer 2

Yes, print is a pure function. The value it returns has type IO (), which you can think of as a bunch of code that outputs the string you passed in. For each string you pass in, it always returns the same code.