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Converting variants with only constant constructors to integers
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Date: 2010-06-08 (08:00)
From: bluestorm <bluestorm.dylc@g...>
Subject: Re: [Caml-list] Converting variants with only constant constructors to integers
> Incidentally, the general function for getting a unique integer for a
> variant type with non-constant constructors isn't that bad either:
> let int_of_gen x =
>  let x = Obj.repr x
>  in
>    if Obj.is_block x
>    then -1 * Obj.tag x - 1
>    else (Obj.magic x : int)

I consider that version much better. You could also write it in a more
restrictive way :

 let to_int x =
   let repr = Obj.repr x in
   if Obj.is_int repr
   then (Obj.obj repr : int)
   else invalid_arg "to_int";;

The good point about those is that they check that the memory representation
is compatible before casting (while the "%identity" does not). They're
safer. Of course, they still break parametricity if used polymorphically, so
as you said type should be constrained.

I still would prefer the more direct pattern-matching definition : it may
require code generation not to be tedious, but the generated code is an
honourable OCaml citizen.