It was implemented in 20 minutes and just for fun.
There is toy AST which compiles to C and gets foreign imported back
to Haskell module. All in Template Haskell.
Run make to build.
Slightly more real-world-ish example would use real prety printing library, some clever monadic syntax as an eDSL, type checking, some code transformations, more clever interface for generating C files, so user can define more than one "DSL2FFI" function. But most likely that would never happen in this toy example.
For something interesting in this area (though it's not a DSL) see https://hackage.haskell.org/package/language-c-inline