The Data.Array.* hierarchy is unsafe (or, Segfaulting for fun
Nils Anders Danielsson
nad at cs.chalmers.se
Tue Dec 5 13:50:28 EST 2006
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
inRange (l,u) i = True
range (l,u) !! index (l,u) i = ⊥ ≠ i.
Furthermore, more seriously, the third law isn't type correct, and a
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..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.
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