Hi Gregory,
First post. I'm a newbie, been using Haskell for about a week and love it. Anyway, this is something I don't understand. Parsers are monadic. I can see this if the parser is reading from an input stream but if there's just a block of text can't you just have the parser call itself recursively feeding the unparsed text down the recursion and tacking nodes onto the tree as the recursions return, finally returning the whole tree to the top level caller. Seems this meets the criteria for pure functionality, same result every time with same args.
Your intuition is right - it's definitely possible to write a parser without using a monad. However, some experience has shown that the most convenient way to structure a recursive descent parser library is with monads (or maybe with arrows, a more general concept). I imagine your suspicion is the result of equating monads with effects. Monads serve many purposes besides dealing with effects elegantly. Many new haskell hackers get their first taste of monads with IO, and get the impression that this is the One True Purpose of monadic programming. You might want to have a look at the paper Monadic Parser Combinators by Graham Hutton and Erik Meijer. This paper serves as a great introduction to both parser combinators and monads, and shows how they are related: http://www.cs.nott.ac.uk/~gmh/bib.html#monparsing The first 10 or 12 pages are particularly helpful in figuring out why monads make sense in parsing. Good luck! --Chris