]> 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 ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners