The Data.Array.* hierarchy is unsafe (or, Segfaulting for fun and profit)

[Simon Marlow]

Nils Anders Danielsson wrote:
> On Tue, 05 Dec 2006, Simon Marlow <simonmarhaskell at gmail.com> wrote:
> 
> 
>>To me, the wording "An implementation is entitled to assume..."
>>implies that there are no obligations on the implementation should the
>>assumption not hold -
>>no obligation to yield _|_ or any other behaviour.
> 
> 
> I guess that's a valid interpretation.
> 
> As an aside, it is interesting to note that the Ix instance for
> Integer does not satisfy the first law (if a wraparound semantics is
> used for Int; this is allowed by the report). With
>   l = 0, u = toInteger (maxBound :: Int) + 1 and i = u
> we have
>   inRange (l,u) i = True
> but
>   range (l,u) !! index (l,u) i = ⊥ ≠ i.
> 
> Furthermore, more seriously, the third law isn't type correct, and a
> corrected version,
> 
>   map (index (l,u)) (range (l,u)) == [0..rangeSize (l,u)],
> 
> isn't satisfied by the Ix instance for Int, since (with
> b = (0, 0 :: Int))
>   map (index b) (range b) = [0]
> while
>   [0..rangeSize b] = [0, 1].
> I think the law should really be
> 
>   map (index (l,u)) (range (l,u)) == [0..rangeSize (l,u) - 1].
> 
> This law is also prone to overflow errors, by the way.

Thanks Nils, I'll make sure these points get addressed during the Haskell' process.

Cheers,
	Simon