The hardest part of combining two languages through a foreign function interface (eg Ruby/C using libffi) is making their garbage collectors work together. You

The hardest part of combining two languages through a foreign function interface (eg Ruby/C using libffi) is making their garbage collectors work together. You basically have to either use a single garbage collector across the language boundary, like the family of languages implemented on the JVM do, or require one or both of the languages to implement GC-less memory management, like in the Ruby/C case.