This sounds like a really interesting question. To save some people weeding through the thread and Jon Harrop's usual trolling garbage, here's a description of the problem:
[quote]
Here's [a]language to to interpret (where postfix * means tupling):
Variables: x
Integer literals: i
Terms:
t = Lambda x*. t
| Apply t t*
| Var(x)
| Num(i)
We assume usual operational semantics of lambda calculus (i.e. static scoping).
The task is to write two interpreters, one with variables x being
DeBruijn indices and one with them being names.
You should go for maximal sharing, i.e. factor out commonalities into
a common class/typeclass/functor/whatever, so that there remains no
duplication of code in the two solutions.
[/quote]