Re: [Caml-list] obj.magic for polymorphic record fields

[Goswin von Brederlow <goswin-v-b@web.de>]

"Damien Guichard" <alphablock@orange.fr> writes:

>> I once faced this situation and the solution is to use modules.
> That is one good practical solution.
>
> The simpler solution that immediatly came to my mind is eta-expansion.
>
>   type foo = {bar : 'a. 'a -> 'a}
>   let a : int -> int = fun x -> x
>   let baz = {bar = fun x -> (Obj.magic a) x}

This would create a new closure though. Wastes time and space. Bad if
you create a million foo's all with the same function.

> However it issues a warning so i acknowledge it's less elegant.

Which I don't quite understand.

> - damien

Why not use magic on the overall type of the record instead of the
individual function?

# type foo = { foo : 'a. 'a -> 'a; }
  type 'a bar = {bar : 'a -> 'a}
  let a : int -> int = fun x -> x
  let baz = (Obj.magic {bar = a} : foo);;

type foo = { foo : 'a. 'a -> 'a; }
type 'a bar = { bar : 'a -> 'a; }
val a : int -> int = <fun>
val baz : foo = {foo = <fun>}

It means duplicating the record type but you can magic it without
warnings.


Overall I have to say though that you are doing something wrong here.
(Should I assume your actual use case is larger than this simple
example?)

# let b x = x + 1
  let buzz = (Obj.magic {bar = b} : foo)
  buzz.foo [1;2;3];;
val b : int -> int = <fun>
val buzz : foo = {foo = <fun>}
- : int list = [-2330612807164231680]

Doesn't always segfault but lets get nastier:

# (buzz.foo buzz).foo 1;;
zsh: segmentation fault  ocaml

The obj.magic only works safely if the function you use is of type 'a
-> 'a (even if the interface restricts it to a less general type). The
right solution would be to loosen the interface to use the more
general type.

MfG
        Goswin