[Caml-list] A shallow option type

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

Hi,

I wish there was an option type that would work without extra
indirection (or more importantly without extra allocation of an ocaml
value when setting it to something).

Why? I'm interfacing with C in a multithreaded way. The data is
allocated on the C side so it won't be moved around by the GC. The ocaml
side will modify data and the C side will utilize it. Now if ocaml
changes a multable 'a option then it will allocate a block for "Some x"
and the GC will move that block around during compaction. Which means
the 'a option can't be safely used without holding the runtime system
lock. Which then means the threads can't run in parallel.

What I want is a

    type 'a shallow = NULL | 'a  (constraint 'a != 'b shallow)

I have some ideas on how to implement this in a module as abstract type
providing get/set/clear functions, which basically means I map None to a
C NULL pointer and Some x to plain x. I know x can never be the NULL
pointer, except when someone creates a 'a shallow shallow and sets Some
None. That would turn into simply None.

Is it possible to somehow declare the constraint that 'a in 'a shallow must
not be itself a 'b shallow?

MfG
        Goswin