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[Caml-list] NaN-safe min and max
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Date: -- (:)
From: Jacques Garrigue <garrigue@k...>
Subject: Re: [Caml-list] NaN-safe min and max
From: Remi Vanicat <vanicat@labri.u-bordeaux.fr>

> 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.

Very interesting: you have found an unsoundness in the ocaml compiler.
Using your example combined with arrays we obtain:

# let arr = [|Lazy.lazy_from_val 0.0; lazy 1.0|];;
val arr : float Lazy.t array = [|lazy 0.; lazy 4.32681603021051e-311|]

i.e., the second lazy is assumed to be a float value, and results in a
wrong result (clearly 1.0 is not anything close to 4.32e-311)

So my take on this is that the code suggested for min is correct, this
is the compiler which is wrong: two values with the same type _must_
have the same representation.

A new headache for implementers...

   Jacques Garrigue

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