What OCaml standard library types cannot be marshalled?

I have a failure Marshaling a data structure (error abstract type (Custom)). There is one known abstract type in use, namely开发者_JS百科 Big_int. However that Marshals fine. There is no custom C code in the application. Apart from Nums, Unix library is also used (however I don't believe there are any active objects of that type). We're Marshal'ing with Closures.

Two (only) third party libraries are in use: OCS Scheme (Scheme interpreter, pure Ocaml) and Dypgen (extensible GLR parser, also pure Ocaml). The problem is with a new feature of Dypgen, saving a dynamically extended parser.

The Ocaml error message is next to useless (it doesn't identify which abstract type with Custom tag is the culprit).

We suspected Lexbuf as the culprit because it contains a closure over an Ocaml channel, and can't be Marshal'ed, but it seems this isn't the problem. So my question is:

Which standard library components can't be Marshall'd?

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Weak arrays cannot be marshaled. I am not familiar with OCS Scheme, but I would expect an interpreter for a garbage-collected language written in OCaml to use weak pointers (they let you piggy-back on OCaml's memory management).

In OCaml's defense, I do not think that the Custom method block contains the name of the type (retrospectively, that seems like a good thing to have).

EDIT: Yep:

$ grep Weak ~/Downloads/ocs-1.0.3/src/*.ml
/Users/pascal/Downloads/ocs-1.0.3/src/ocs_sym.ml:module SymTable = Weak.Make (HashSymbol)

EDIT2: As pointed out by ygrek, there is room for a name in the custom method block. I should also clarify that weak arrays are not custom values, since my answer seemed to imply that. Weak arrays have the Abstract tag and are chained using the first word of data so that the garbage collector can traverse them in special weak-pointer-related phases of the collection cycle.