After Lőri posted his C++ template meta-program solution to the 9-digit problem, Encsé asked me to help him understand it, being the first real-world metaprogram he's encountered. To help both him and myself, I rewrote, function-by-function, his code into Haskell.
This rewriting process was surprisingly straightforward, and the resulting code, while being in a very strict one-to-one correspondence with the original C++ code, was immensely more readable. So this, naturally, lead to the idea of a simple functional programming language that could be compiled into C++ compile-time metaprograms.
A couple of days later, I had a small accident and had to stay in bed for a week, which gave me plenty of time to throw together a proof-of-concept compiler from a Haskell-like language into C++ TMP.
And so MetaFun was born.
MetaFun is structured into three separate modules, one for parsing and typechecking the input language called Kiff (for Keep It Fun & Functional), one for unparsing C++ metaprograms, and the third one is, of course, the compiler itself. I plan to re-use the Kiff module whenever I come up with an idea for another new throwaway functional compiler.