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

[David House <dhouse@janestreet.com>]

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My attachment got rejected by the list. http://imgur.com/de9SBnA


On 17 January 2014 09:09, David House <dhouse@janestreet.com> wrote:

> Here's some high-tech computer visualisation to illustrate this.
>
>
> On 17 January 2014 08:40, David House <dhouse@janestreet.com> wrote:
>
>> 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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