How to determine the directory in which a running Haskell script or application lives?

Question

I have a Haskell script that runs via a shebang line making use of the runhaskell utility. E.g...

#! /usr/bin/env runhaskell
module Main where
main = do { ... }

Now, I'd like to be able to determine the directory in which that script resides from within the script, itself. So, if the script lives in /home/me/my-haskell-app/script.hs, I should be able to run it from anywhere, using a relative or absolute path, and it should know it's located in the /home/me/my-haskell-app/ directory.

I thought the functionality available in the System.Environment module might be able to help, but it fell a little short. getProgName did not seem to provide useful file-path information. I found that the environment variable _ (that's an underscore) would sometimes contain the path to the script, as it was invoked; however, as soon as the script is invoked via some other program or parent script, that environment variable seems to lose its value (and I am needing to invoke my Haskell script from another, parent application).

Also useful-to-know would be whether I can determine the directory in which a pre-compiled Haskell executable lives, using the same technique or otherwise.

Answer 1

For compiled executables, In GHC 7.6 or later you can use System.Environment.getExecutablePath.

getExecutablePath :: IO FilePathSource

  Returns the absolute pathname of the current executable.
  Note that for scripts and interactive sessions, this is the path to the
  interpreter (e.g. ghci.)