[Caml-list] Differentiating lists from arrays with Obj

[John Goerzen <jgoerzen@complete.org>]

Hello,

I am looking at using the Obj to access internal representations of some
types.

I have figured out how to access those types for ints, doubles (floats),
strings, and arrays of doubles (why those are special, I have no idea!).

I can also figure out how to access lists and arrays, for which Obj.tag
returns zero.  (Although that value does not occur in obj.ml, it is in
some of the header files.)

Now, here's the question: how can I accurately differentiate lists from
arrays?

Lists appear to be internally represented by structures that look
somewhat like this:

[1; [2; [3; 0]]]

That is, Obj.size will return 2 for the list [1; 2; 3] because it is a
collection of an int and a list.  That's fine, but its internal
representation of the list [1] is:

[1; 0]

Which happens to be identical to the representation of the array
[| 1; 0 |].  Eep.  Now, it's not a big deal to me whether I've got a
list or an array, but I need to be able to know whether that list or
array has an extra 0 at the end.

Worse, the internal representation of [] appears to be identical to the
internal representation of the integer 0.

I am a little puzzled here, and I do not know the type of the data I'm
dealing with in advance.

Thanks for any enlightenment.

-- John

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