[Caml-list] Quick question - accessing variant value from C

[Caml-list] Quick question - accessing variant value from C

[David Fox]

I have a value of the following type being passed to an external:

  type pointer = Null | Address of string

and I'd like to determine (in the C code) which variant the value is
and get the pointer to the string.  I've stared at mlvalues.h for a
while, but I can't really figure it out.  Can anyone help?  You won't
regret it...
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Re: [Caml-list] Quick question - accessing variant value from C

[Jean-Christophe Filliatre]

Hi,

You can use the macros Is_long / Is_block to discriminate between Null
and Address, since the first one  is represented as an integer and the
second one  as a  structured block  (in the later  case, you  can then
access the string with the Field macro).

Hope this helps,
-- 
Jean-Christophe FILLIATRE
  mailto:Jean-Christophe.Filliatre@lri.fr
  http://www.lri.fr/~filliatr

David Fox writes:
 > I have a value of the following type being passed to an external:
 > 
 >   type pointer = Null | Address of string
 > 
 > and I'd like to determine (in the C code) which variant the value is
 > and get the pointer to the string.  I've stared at mlvalues.h for a
 > while, but I can't really figure it out.  Can anyone help?  You won't
 > regret it...
 > -------------------
 > To unsubscribe, mail caml-list-request@inria.fr.  Archives: http://caml.inria.fr
-------------------
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Re: [Caml-list] Quick question - accessing variant value from C

[Sven LUTHER]

On Thu, Mar 29, 2001 at 09:54:02AM +0200, Jean-Christophe Filliatre wrote:
> 
> Hi,
> 
> You can use the macros Is_long / Is_block to discriminate between Null
> and Address, since the first one  is represented as an integer and the
> second one  as a  structured block  (in the later  case, you  can then
> access the string with the Field macro).

Additionally, some reading of the "Interfacing C with Objective Caml" chapter
of the ocaml documentation is a must read for this, far more inmportant than
the mlvalues.h file, i think.

Friendly,

Sven Luther
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