I have written a few functional programming languages built on lambda calculus based backends, but I've had problems with compiling it efficiently; it's hard (at least to me)! I find that combinator based compilers are difficult, but very elegant; I've written a few in the past but problems always spring up when trying to evaluate expressions properly AND do side-effects correctly. I would love to pursue writing a combinator compiler, but I haven't yet come up with a good system of combinators that can represent I/O and foreign functions properly; although this probably isn't even as difficult as writing a compiler to a Turing-tape architecture.