[Caml-list] Problem with universal functions in a module

[Goswin von Brederlow <goswin-v-b@web.de>]

Hi,

I have a functor that boxes a GADT type into an universal container.
But then the problem is how to get the type back out again.
Specifically I want to pass the data to a polymorphic function that
doesn't care what type it gets since it will pattern match the GADT to
determine the right type.

But, because the argument is a GADT, ocaml complains after unboxing
the type would escape its scope (see call_unboxed below). One can only
call an universal function with the unboxed data and only records (and
objects) can have universal functions.

Problem is that I would like to keep the implementation of the helper
record abstract and only provide a constructor (helper function
below). But the constructor needs an universal function and function
argument are only polymorphic (less general).

A) why is a polymorphic function less general here?

The print function is good enough for List.iter below or for
constructing a helper but not indirectly via "M.helper fn".

Why are function arguments less general than their original function?

B) is there some way around this I'm not seeing?

MfG
	Goswin

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