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

Nils Anders Danielsson wrote:

On Tue, 05 Dec 2006, Simon Marlow 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