Introduction to Programming
                   Assignment 5, 5 Nov 2008
                      Due Mon 17 Nov 2008

Note: Remember to always supply a type definition for every
      function you write, including any auxiliary functions that
      you may define to solve the given problem.

			      grep

grep is a useful unix command to identify lines in a text file
that match a pattern.  In its most basic form, grep will take a
fixed string of characters (a pattern) and a file name, and
report all lines in the file that contain that pattern, prefixed
by the line number.  A typical example is shown here.

  $ grep memofib lecture19-03nov2008.txt
  252:  fib n = memofib n []
  254:The function memofib takes an argument for which the function is
  258:  memofib :: Int -> [(Int,Int)] -> (Int,[(Int,Int)])
  259:  memofib n l 
  280:          (vala,memoa) = memofib (n-1) l
  281:          (valb,memob) = memofib (n-2) memoa
  382:  fib n = memofib n emptytable
  384:  memofib :: Int -> [Table Int Int] -> (Int,Table Int Int)
  385:  memofib n t
  391:          (vala,memoa) = memofib (n-1) l
  392:          (valb,memob) = memofib (n-2) memoa
  398:in the definition of fib/memofib.

(Note: In unix, you actually have to say "grep -n" to get the
       line numbers, ...).

Write a Haskell program that does the following:

1. Ask the user for the string to use as a pattern.
2. Ask the user for the name of an input file.
3. Print out the line number and contents of each line that
   contains the pattern string, similar to the example above.

IMPORTANT: When you use "putStr" to prompt the user for input in
           steps 1 and 2, terminate your string with "\n".

Your program should additionally have the following features:

1. It should compile into a standalone program using the compiler
   "ghc".  Submit your code in two modules:

   Main.hs  -- contains just one function "main"
   Grep.hs  -- contains all the other functions required
               to solve the assignment

2. Your program should handle the following exceptions:

   a. If the input file specified by the user does not exist,
      your program should print "Input file does not exist" and
      exit.

   b. All other exceptions can be passed on to a higher level
      using the function "ioError".

   "File does not exist" is signalled by the exception

      isDoesNotExistError :: IOError -> Bool

======================================================================