Change exit value on getLine exception in Haskell

Question

I'm working on a student project in Haskell and i'm having a problem with getLine's behaviour. Here's the code (simplified) :

main :: IO()
main = do
str <- getLine
putStrLn str

What i'd like to do is, when the user presses Ctrl+D, be able to exitWith (ExitFailure 84). getLine simply prints an error and exit the program (and returns 1)

deBruijn: <stdin>: hGetLine: end of file

How to change this behaviour ? I only want to change the exit value to 84.

Answer 1

Your program never sees Control-D. What it does see is the fact that standard input has been closed, in this case by your terminal in response to Control-D being typed. This means you want to catch the EOF condition before getLine tries to read a line from a closed file.

import System.IO
import System.Exit

main :: IO ()
main = do
    isClosed <- isEOF
    if isClosed 
      then exitWith (ExitFailure 84)
      else getLine >>= putStrLn

Answer 2

Instead of manually checking for isEof you could just catch the IO exception as it happens:

import Control.Exception (catch)
import System.IO.Error(isEOFError)
import System.Exit

tryMain :: IO ()
tryMain = getLine >>= putStrLn

main :: IO ()
main = tryMain `catch` (\e ->
  if isEOFError e
  then exitWith (ExitFailure 84)
  else exitWith (ExitFailure 99))

As you can in general not rule out IO exceptions in advance, this is the approach I'd recommend.