extending records with Obj.magic

extending records with Obj.magic

[Nicolas Ojeda Bar]

Hello,

I need extensible records, and the code below seems to
work. Are there any pitfall that I should be aware of?
Could this mess up the GC?

# type t0 = { a : int };
# type t1 = { a2 : int; b : int };
# value x1 = { a2 = 3; b = 5 };
# value x0 : t0 = Obj.magic x1;
value x0 = { a = 3 }
# value x0' : t1 = Obj.magic x0;
value x0' = { a2 = 3; b = 5 }

(supposedly t1 is an extension of t0). The types are
being generated by a program, so I am not worried about
actually having to _write_ this myself.

Thanks!
N

Re: [Caml-list] extending records with Obj.magic

[bluestorm]

On Mon, Aug 30, 2010 at 11:04 PM, Nicolas Ojeda Bar
<nojb@math.harvard.edu> wrote:
> Hello,
>
> I need extensible records, and the code below seems to
> work. Are there any pitfall that I should be aware of?
> Could this mess up the GC?
>
> # type t0 = { a : int };
> # type t1 = { a2 : int; b : int };
> # value x1 = { a2 = 3; b = 5 };
> # value x0 : t0 = Obj.magic x1;
> value x0 = { a = 3 }
> # value x0' : t1 = Obj.magic x0;
> value x0' = { a2 = 3; b = 5 }
>

You could use objects instead.

ype t0 = < a : int >
type t1 = < a : int; b : int >

let x1 = object
  method a = 3
  method b = 5
end

let x0 = (x1 :> t0)

(downcasting x0 -> x0' isn't correct)

Re: [Caml-list] extending records with Obj.magic

[Martin Jambon]

Nicolas Ojeda Bar wrote:
> Hello,
> 
> I need extensible records, and the code below seems to
> work. Are there any pitfall that I should be aware of?
> Could this mess up the GC?
> 
> # type t0 = { a : int };
> # type t1 = { a2 : int; b : int };
> # value x1 = { a2 = 3; b = 5 };
> # value x0 : t0 = Obj.magic x1;
> value x0 = { a = 3 }
> # value x0' : t1 = Obj.magic x0;
> value x0' = { a2 = 3; b = 5 }
> 
> (supposedly t1 is an extension of t0). The types are
> being generated by a program, so I am not worried about
> actually having to _write_ this myself.


# type t0 = { a : float };;
type t0 = { a : float; }
# type t1 = { a2 : float; b : int };;
type t1 = { a2 : float; b : int; }
# let x1 = { a2 = 3.; b = 5 };;
val x1 : t1 = {a2 = 3.; b = 5}
# let x0 : t0 = Obj.magic x1;;
val x0 : t0 = {a = 3.}

And now the magic:

# x0.a;;
- : float = 6.94364726476075e-310



Martin

-- 
http://mjambon.com/

Re: [Caml-list] extending records with Obj.magic

[Bruno Barras]

On 08/30/2010 11:04 PM, Nicolas Ojeda Bar wrote:
> Hello,
>
> I need extensible records, and the code below seems to
> work. Are there any pitfall that I should be aware of?
> Could this mess up the GC?
>
> # type t0 = { a : int };
> # type t1 = { a2 : int; b : int };
> # value x1 = { a2 = 3; b = 5 };
> # value x0 : t0 = Obj.magic x1;
> value x0 = { a = 3 }
> # value x0' : t1 = Obj.magic x0;
> value x0' = { a2 = 3; b = 5 }

You should also be aware that structural equality (=) is untyped, so you 
might get surprised by this:

type t0 = { a : int; };;
# type t1 = { a2 : int; b : int; }
type t1 = { a2 : int; b : int; };;
# let x = {a2=3; b=1};;
val x : t1 = {a2 = 3; b = 1}
# let y = {a2=3; b=2};;
val y : t1 = {a2 = 3; b = 2}
# let x0 : t0 = Obj.magic x;;
val x0 : t0 = {a = 3}
# let y0 : t0 = Obj.magic y;;
val y0 : t0 = {a = 3}
# x0 = y0;;
- : bool = false

x0 and y0 look structurally equal, but they're not!

The same oddity will show up with Pervasives.compare and 
Pervasives.hash, just to mention those two.

Bruno.

Re: [Caml-list] extending records with Obj.magic

[Goswin von Brederlow]

Martin Jambon <martin.jambon@ens-lyon.org> writes:

> Nicolas Ojeda Bar wrote:
>> Hello,
>> 
>> I need extensible records, and the code below seems to
>> work. Are there any pitfall that I should be aware of?
>> Could this mess up the GC?
>> 
>> # type t0 = { a : int };
>> # type t1 = { a2 : int; b : int };
>> # value x1 = { a2 = 3; b = 5 };
>> # value x0 : t0 = Obj.magic x1;
>> value x0 = { a = 3 }
>> # value x0' : t1 = Obj.magic x0;
>> value x0' = { a2 = 3; b = 5 }
>> 
>> (supposedly t1 is an extension of t0). The types are
>> being generated by a program, so I am not worried about
>> actually having to _write_ this myself.

You need to make sure t1 is structurally an extension of t0 and then it
will be safe to use in the current implementation of the GC (see counter
example below). As others have said beware of functions that do
structural compares on the magiced values. But GC compaction will keep
the value intact as it only looks at the in-memory structure and
Obj.magic doesn't change that.

> # type t0 = { a : float };;
> type t0 = { a : float; }
> # type t1 = { a2 : float; b : int };;
> type t1 = { a2 : float; b : int; }
> # let x1 = { a2 = 3.; b = 5 };;
> val x1 : t1 = {a2 = 3.; b = 5}
> # let x0 : t0 = Obj.magic x1;;
> val x0 : t0 = {a = 3.}
>
> And now the magic:
>
> # x0.a;;
> - : float = 6.94364726476075e-310

This is a special case because records with ONLY floats are stored as
unboxed float array. Mixed records on the other hand use boxed
floats. So in you example t0 is structurally not a prefix of t1.

MfG
        Goswin