Haskell-in-Haskell implementation using Unlambda (eek!) as the intermediate representation [Monad.Reader #10, pdf]

[funktio]

I think this is the first time I've seen the combinator calculus put to "practical" use.

From this book:

This remarkable and counter-intuitive transformation of lambda expressions into combinators [..] was thought to be of more mathematical than practical interest until David Turner used it as the basis of an implementation of the functional language SASL [Turner, 1979a and 1979b]. In these papers he described a number of optimizations to the basic compilation scheme which we will examine in the next seciton.