What this is:

It's a slightly cleaned-up version of some example code from an answer on Stack Overflow. It's mostly a silly example of doing some computation with type families--specifically, to construct terms in a type-directed manner, rather than constructing free-floating types for their own sake as is often used for examples.

What it does:

Insofar as it does anything terribly useful, it converts between functions using standard Haskell types, e.g. Int -> String -> IO Double, and equivalent functions using C types for the FFI, e.g. CInt -> CString -> IO CDouble.

Lots of functionality is missing, though, since it was written as a demonstration.

What else?

You'll need a recent version of GHC to try this. And by recent, I mean "released a few days before I wrote this code", heh.