Homework code for the "Hoodlums" London Haskell Meetup group

201010_Parser:
 - Task: Implement a parser for a simple expression tree.
 - Contents:
   - Expressions.hs:
       The expression tree implementation, some demonstration catamorphisms.
   - MonadicParser.hs:
       A parser combinator library implementation, using a monadic style.  
       Based on code from the October Hoodlums meeting and from the 1998 paper 
       "Monadic parsing in Haskell" by Hutton and Meijer.  Running a parser
       returns a list of possible results; an empty list indicates an error.
   - ApplicativeParser.hs:
       A parser combinator library implementation, using an applicative style.
       NB: uses a different parser model than MonadicParser.hs --- returns an
       "Either", with left indicating an error (Left (err, remaining string)), 
       right indicating a successful parse (Right (result, remaining string)).
   - MonadicExpressionParser.hs, ApplicativeExpressionParser.hs:
       Parsers for the expression language in "Expressions.hs", using the
       Monadic or Applicative style.
   - main.hs:
       A simple program to test the above; takes a string from the user and
       attempts to parse using the monadic and applicative parsers, doing some 
       simple error checking.
       - compile with ghc --make main