"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.
-- 
Rémi Vanicat
remi.vanicat@laposte.net

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