Call main function with arguments

Question

I have problem with reading from file. Whenever I need to read from a file, I do something like this:

main = do x <- readFile "/tmp/foo.txt"
      putStr x

But now I would like the path to be an argument, so I tried the following

main s = do x <- readFile s
        putStr x

It doesn't work. I see the following error:

Couldn't match expected type `IO t0'
            with actual type `FilePath -> IO ()'
In the expression: main
When checking the type of the function `main'

My homework is to write a program and the program has to contain a main function (because it will be compiled) and argument of call must contain the name of the file. I'm not sure I understand this and I don't know how to proceed. I will grateful for some help.

Answer 1

The Haskell report specifies that the main function always has type IO t, (for some type t which will be ignored) and hence never takes normal function arguments, so this is not the right approach.

You are looking for the function getArgs (for which you have to import the module System.Environment. It returns the arguments passed to your program as a list of Strings.

So your code would look like:

import System.Environment
main = do
    args <- getArgs
    case args of 
      [file] -> do
        x <- readFile file
        putStr x
      _ -> putStrLn "Wrong number of arguments"

Answer 2

In Haskell, the arguments are NOT given to the main function because of the way Haskell binds its start up and to remain consistent. You need to use System.Environment.getArgs.

In particular, because Haskell is a pure functional language, main is a monadic action that organizes the side-effect-ful computations performed by the software – the result computed by main is discarded, because in functional languages you are detached from the environment w.r.t. computation and only interact with it as a side-effect.

Example

import System.Environment

main = do x <- getArgs; print x

This will print out whatever you type on the command line.

The Haskell wiki has an excellent tutorial on the topic.