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Context: I have a program with a variant type named "value". ( See here: https://gist.github.com/anonymous/6390fa6d9cb0de2c5ef3 ). I want to find a way of taking a dynamically constructed definition of a C function, then loading that C function using CTypes "foreign" and wrapping the result with a converter function that would, for example, translate an int->string->int to a value->value->value.

This is a working implementation of what I want with 2 arguments. The code:
https://bitbucket.org/runhello/test-emily-game/src/e1cac5bf0546234ab7b69977b53625efaef7d8d7/src/internalPackage.ml?at=default

When I start moving to more than 2 arguments-- before I even get more than 2 arguments working!-- I find something I don't understand involving existential types. The code:
https://bitbucket.org/runhello/test-emily-game/src/6ea499b99d92f1f507e95d3eee7ebdc2fd4244ab/src/internalPackage.ml?at=default

When I compile this, I get this error message. I don't understand it. My question: Why is cReturnType expected?

Andis-Macbook:test-emily-game mcc$ makeocamlbuild -no-links -use-ocamlfind  src/main.native.o
Finished, 0 targets (0 cached) in 00:00:00.
+ ocamlfind ocamlc -c -safe-string -package 'ctypes, ctypes.foreign' -package 'sedlex, containers, fileutils, ppx_getenv, ppx_const, uutf' -I src -o src/internalPackage.cmo src/internalPackage.ml
File "src/internalPackage.ml", line 63, characters 28-58:
Error: This expression has type cChainedReturnType#0
       but an expression was expected of type cReturnType#0
       Command exited with code 2.
       Compilation unsuccessful after building 22 targets (21 cached) in 00:00:00.
       make: *** [_build/src/main.native.o] Error 10