Question

Ok, guys, super easy question (it seems weird that Google didn’t help me with this one):

import IO

--.... yadda, yadda, yadda

  file <- openFile "/some/path" ReadMode

How do I check if the handle that I got from openFile is a valid handle, i.e. that the file exists and was opened successfully?

Answer 1

If the file doesn’t exist, or some other error occurs, the call to openFile will fail with an exception.

For example:

import System.IO

main = do
   openFile "/some/path" ReadMode

Fails with:

A.hs: /some/path: openFile: does not exist (No such file or directory)

The types of exception that may be thrown by openFile are listed here, and include:

• isAlreadyInUseError if the file is already open and cannot be reopened;
• isDoesNotExistError if the file does not exist; or
• isPermissionError if the user does not have permission to open the file.

You can catch these errors using Control.Exception, like so:

{-# LANGUAGE ScopedTypeVariables #-}

import System.IO
import Control.Exception

main = do
    handle (\(e :: IOException) -> print e >> return Nothing) $ do
      h <- openFile "/some/path" ReadMode
      return (Just h)