Remove file if it exists

Question

What is the right way of doing this in Haskell?

if exists \"foo.txt\" then delete \"foo.txt\"
doSomethingElse

So far I have:

im         


        

Answer 1

(Note: ehird's answer makes a very good point regarding a race condition. It should be kept in mind when reading my answer, which ignores the issue. Do note also that the imperative pseudo-code presented in the question also suffers from the same problem.)

What defines the filename? Is it given in the program, or supplied by the user? In your imperative pseudo-code, it's a constant string in the program. I'll assume you want the user to supply it by passing it as the first command line argument to the program.

Then I suggest something like this:

import Control.Monad    
import System.Directory
import System.Environment

doSomethingElse :: IO ()

main = do
  args <- getArgs
  fileExists <- doesFileExist (head args)
  when fileExists (removeFile (head args))
  doSomethingElse

(As you can see, I added the type signature of doSomethingElse to avoid confusion).

I import System.Environment for the getArgs function. In case the file in question is simply given by a constant string (such as in your imperative pseudo-code), just remove all the args stuff and fill in the constant string wherever I have head args.

Control.Monad is imported to get the when function. Note that this useful function is not a keyword (like if), but an ordinary function. Let's look at its type:

when :: Monad m => Bool -> m () -> m ()

In your case m is IO, so you can think of when as a function that takes a Bool and an IO action and performs the action only if the Bool is True. Of course you could solve your problem with ifs, but in your case when reads a lot clearer. At least I think so.

Addendum: If you, like I did at first, get the feeling that when is some magical and difficult machinery, it's very instructive to try to define the function yourself. I promise you, it's dead simple...