Remi Vanicat <vanicat@labri.u-bordeaux.fr> writes:

> "Yaron Minsky" <yminsky@cs.cornell.edu> writes:
>
>> Olivier Andrieu proposed the following code for doing a safe min/max. 
>> Does anyone have any thoughts about this code snippet, and whether it is
>> or is not a safe use of Obj.magic?
>>
>>  let min a b =
>>     let o = Obj.repr a in
>>     if Obj.is_block o && Obj.tag o = Obj.double_tag
>>     then Obj.magic (fmin (Obj.magic a) (Obj.magic b))
>>     else Pervasives.min a b
>
> this one is unsafe : the fact that a is a block with a tag
> Obj.double_tag doesn't proof that the same is true for b even if a and
> b have the same type :
>
> # let i1 = Lazy.lazy_from_val 0.0;;
> val i1 : float Lazy.t = lazy 0.
> # let o1 = Obj.repr i1;;
> val o1 : Obj.t = <abstr>
> # Obj.is_block o1;;
> - : bool = true
> # Obj.tag o1 = Obj.double_tag;;
> - : bool = true
> # let i2 = lazy 3;;
> val i2 : int lazy_t = <lazy>
> # let o2 = Obj.repr i2;;
> val o2 : Obj.t = <abstr>
> # Obj.is_block o2;;
> - : bool = true
> #  Obj.tag o2 = Obj.double_tag;;
> - : bool = false

> both i1 and i2 have the same type, but only the first one is a block of
> tag Obj.double_tag. A verification have to be done for both
> argument.

If you replace 
let i2 = lazy 3;;
by
let i2 = lazy 3.;;

then what I've said became true.

[...]


-- 
Rémi Vanicat
remi.vanicat@laposte.net

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