Re: [Caml-list] Converting variants with only constant constructors to integers

["Török Edwin" <edwintorok@gmail.com>]

On 06/07/2010 09:31 PM, Török Edwin wrote:
> On 06/07/2010 09:23 PM, Wojciech Meyer wrote:
>> Török Edwin <edwintorok@gmail.com> writes:
>>
>>> What is the recommended way to convert a variant that has only constant
>>> constructors to an integer? (an integer that is unique for each constant
>>> constructor, preferably sequential in the order in which they are declared).
>>
>> In the first play you'd need to tell us, why would you need this
> 
> Yes, I probably should have started with that.
> I have some flags like:
> type myflags = Property1 | Property2 | Property3;;
> 
> I have something similar code in C, as an enum.
> I need to convert to convert OCaml lists of flags to an integer, that
> will ultimately be passed to a C function.
> 
> So [Property1; Property3] needs to become (1 << Property1) | (1 <<
> Property3). I want to do this conversion in OCaml code.
> 
> Important is that "Property1" from OCaml gets the same value as
> Property1 in the C enum.
> 
>> , and
>> then maybe there are other solutions to your problem.
>> Most likely Marshal module from stdlib, will full-fill your requirements.
> 
> If I only had OCaml code to deal with yes, but see above.
> 
>>
>> I can imagine you could use an association list, function with pattern
>> matching
> 
> I don't have too many flags, so I could write a simple pattern matching
> function to convert to int (even manually).
> I just thought there is a way to do this easily, and more general than that.
> 
>> , code generation tool (based on CamlP4), etc.
>>
>> The later would be needed if you have varied data structure, that change
>> frequently or it is too big to maintain it with just pattern matching.
>>
>>>
>>> I found the following two possibilities, but I am not sure if these are
>>> guaranteed to return the same results for future versions of OCaml or not:
>>>
>>> let int_of_constant_variant a : int = Hashtbl.hash a;;
>>> let int_of_constant_variant a : int =
>>>  let r = Obj.repr a in
>>>   assert (Obj.is_int r);
>>>   (Obj.magic r : int);;
>>
>> There is no guarantee of these kind of unsafe casts at all (means using
>> Obj.magic). Especially Obj.magic should not be used in marshaling, but
>> most common patter is for type system tricks.
> 
> How about the hash value? I guess no guarantees about that either.
> 
> Best regards,
> --Edwin