right, the use case is totally wrong
I'm automatically generating code from type declarations using camlp4 and it
would break down in presence of polymorphic record fields.

Here's the fix I chose (Thanks to Mr. Pouillard):

module type MagicSignature =
sig
  val x : 'a
end

and I would replace every occurence of

{bar = x}

with

let module X : MagicSignature = (struct let y = x end) in {bar = X.y}

-Jacques L.

On Mon, Dec 21, 2009 at 9:00 AM, Boris Yakobowski <boris@yakobowski.org>wrote:

> On Mon, Dec 21, 2009 at 2:44 PM, Goswin von Brederlow <goswin-v-b@web.de>
> wrote:
> >> However it issues a warning so i acknowledge it's less elegant.
> >
> > Which I don't quite understand.
>
> The warning is based on the results of the type inference algorithm.
> You're not supposed to find values of type 'a. 'a "in the wild", yet
> (Obj.magic a) has this type. Thus the Ocaml compiler deduces that
> you're doing something wrong: your function never returns, it does not
> use its arguments, etc. Here, using Obj.magic breaks the crystal ball
> of the compiler, and the warning is incorrect.
>
> There is an easy way to silence the warning though:
>
> let baz = {bar = fun x -> (Obj.magic a : _ -> _) x}
>
> (but not let baz = {bar = fun x -> (Obj.magic a : 'a -> 'a) x} for
> ugly reasons...)
>
> You are however quite correct. Using Obj.magic in this particular case
> seems quite wrong.
>
> --
> Boris
>
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