Re: [Caml-list] How much optimized is the 'a option type ?

[David House <dhouse@janestreet.com>]

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Err, right, sorry. If you have None in, say, a record, that's not allocated
at all. Rather than there being a pointer in that field, there is special
word in that field which represents None.

If that field is a Some, then it's a pointer to a two word allocated block
which in turn points at the actual thing. So compared to a C pointer, there
an extra two words and one more indirection.


On 17 January 2014 08:16, Julien Blond <julien.blond@gmail.com> wrote:

> > An option value always takes two words: one for the header, and then
> either a pointer or a word that means "None".
>
> No. From the reference manual § 19.3.4 :
>
> type 'a option = None           (* Val_int(0), i.e. just an integer value
> = 1 word *)
>                      | Some of 'a   (* block of size 1 = [(header = 1
> word) + (1 field = 1 word)] = 2 words *)
>
>
> 2014/1/17 David House <dhouse@janestreet.com>
>
>> It behaves identically to that type.
>>
>> It is just like any other sum type. However, due to the way that sum
>> types are represented in memory, it is not that inefficient. The only thing
>> that makes it less efficient than a C pointer is the header block
>> (necessary for the GC). An option value always takes two words: one for the
>> header, and then either a pointer or a word that means "None".
>>
>>
>> On 17 January 2014 07:35, Damien Guichard <alphablock@orange.fr> wrote:
>>
>>> Hello,
>>>
>>> Compared to the code :
>>>
>>> type 'a option = None | Some of 'a
>>>
>>> How do an 'a option value performs ?
>>> Any allocation saved ?
>>> Any indirection removed ?
>>>
>>> Is 'a option just like any sum type ?
>>> Or is 'a option more like an ANSI C pointer type ?
>>>
>>> Regards,
>>>
>>> Damien Guichard
>>>
>>>
>>>
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>>
>>
>

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