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From: Wojciech Meyer <wojciech.meyer@googlemail.com>
To: Goswin von Brederlow <goswin-v-b@web.de>
Cc: caml-list@inria.fr
References: <87fwcx6ejm.fsf@frosties.localnet>
Date: Sat, 24 Mar 2012 18:45:02 +0000
In-Reply-To: <87fwcx6ejm.fsf@frosties.localnet> (Goswin von Brederlow's
	message of "Sat, 24 Mar 2012 19:26:37 +0100")
Message-ID: <wfk429dej5.fsf@gmail.com>
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Subject: Re: [Caml-list] Wish: mutable variant types, equivalence with records

Goswin von Brederlow <goswin-v-b@web.de> writes:

> Hi,
>
> consider the code below that counts how often a value is printed.
>
> The reason why this code works is that the memory layout of a
> variant with arguments is identical to a record with the same
> types. The only difference is that a variant sets the tag of the memory
> block to reflect which constructor it is (here Bar = 0, Baz = 1).
>
> But the code is fragile. It requires the use of Obj.magic and duplicates
> the definitions of bar and baz (once as variant and once as record). If
> the type of foo is changed but not the records then bad things will
> happen.
>
> There are ways to do this without Obj.magic:
>
> type foo = Bar of bar_record | Baz of baz_record
> type foo = Bar of int * int ref | Baz of float * int ref
>
> The first adds an indirection for every access to foo and breaks
> matching. The second adds an indirection for every mutable value in the
> type. In both cases the extra indirections increase the memory footprint
> and runtime.
>
>
> So why not allow mutable in variant types and some equivalence with
> records? For example:
>
> type <name> = <Constructor> of [mutable] <type>[:label] [| ...]

Please see [1], Alain Frisch has been working recently on implementing
in-line records for constructor arguments.

It's more implementation/design implications than people might think.

[1] http://caml.inria.fr/mantis/view.php?id=5528

-- Wojciech