Why does the main function in Haskell not have any parameters?

Question

I'm somewhat new to Haskell. I've worked through one ore two tutorials, but I don't have much experience.

Every function in Haskell is pure, this is, why we can't have any I/O without the IO-monad. What I don't understand is, why do the program parameters have to be an IO action as well? The parameters are passed to the program like to a function. Why can't the parameters be accessed like in a function?

To make things clear, I don't understand, why the main function has to look like this

main :: IO()
main = do
    args <- getArgs
    print args

Instead of this

main :: [String] -> IO()
main args = do
    print args

I can't see any reason for it, and I haven't found an answer googling around.

Answer 1

It's a language design choice. Neither approach is strictly better than the other one.

Haskell could have been designed to have a main of either kind.

When one does need the program arguments, it would be more convenient to have them passed as function arguments to main.

When one does not need the program arguments, having them passed to main is slightly cumbersome, since we need to write a longer type, and an additional _ to discard the [String] argument.

Further, getArgs lets one access the program arguments anywhere in the program (inside IO), while having them passed to main, only, can be less convenient since one would then be forced to pass them around in the program, which can be inconvenient.

(Short digression) For what it's worth, I had a similar reaction to yours a long time ago when I discovered that in Java we have void main() instead of int main() as in C. Then I realized that in most programs I always wrote return 0; at the end, so it makes little sense to always require that. In Java that's the implicit default, and when we really need to return something else, we use System.exit(). Even if that is the way it's done in a previous language (C, in this case), new languages can choose a new way to make available the same functionality.