Some time ago I made a little experiment and implemented an interpreter for the lambda-calculus in Haskell. The only reason was that I wanted to try different ways of doing it. So I did it using simple substitution (i.e., as the textbooks describe it), unique identifiers, deBruijn indicies, and higher order abstract syntax. You can get the code from http://darcs.augustsson.net/Darcs/Lambda/ or just check out small paper at http://darcs.augustsson.net/Darcs/ Lambda/top.pdf -- Lennart