main 函数在 haskell 中不接受任何参数

Question

Hi I want to define a main function in haskell, similar to the main-function in java

main :: String -> IO()
main args = do
            putStrLn args

But this gives me an error when I try to run the program.

Answer 1

An Haskell program's entry point has the type of IO () , not String -> IO () .

To get the command-line arguments, use System.Environment 's getArgs .

Using do-notation:

import System.Environment (getArgs)

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

(Note that print is used instead of putStrLn , since args have the type of [String] , and not String .)

Answer 2

I also found this very useful.

main :: IO ()
main =
    do
        input <- getContents
        solve input