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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