At the beginning of Robert Harper's excellent book Programming in Standard ML he presents us with a simple regexp engine.

I am currently writing a version of that engine in Haskell. What's worse, I'm using a monad instead of what was used in the book (a form of continuation passing)! Go, go, monads!

I'll post this code when I get done with it.

Last night I wrote a little lambda-calculus evaluator in Haskell. It only does beta-reduction, and is inadequate because it does not properly alpha-substitute terms which are bound in a lambda-expression but free in what that lambda-expression may be applied to. Hence, you won't see it here now.

Needless to say, I am enjoying getting into Haskell