The Ix instance for (,) leaks space in range

(at least if you take leak to mean unexpected space behaviour).

This was brought to my attention via http://stackoverflow.com/questions/12780497/puzzling-memory-behavior-in-haskell where someone created a 6×10^6-array and was surprised that showing the array caused an additional (3+2)*8*10^6 bytes to be used. The reason (as far as I could tell) was this code:

instance (Ix a, Ix b) => Ix (a, b) where
    range ((l1,l2),(u1,u2)) = [ (i1,i2) | i1 <- range (l1,u1), i2 <- range (l2,u2) ]

in Arr.GHC. The result of range (l2,u2) is shared between every step in the first component of the index.

I guess it is reasonable to expect that the result of range is never worth sharing (is it?). I am not entirely sure what the best, cleanest, alternative implementation is, though. (At least not without a working dontshare annotation ;-))