in traditional Sun javac, you get something that basically looks like:

for (int i=0; i<1000; i++) { if i outside of arr bounds, throw exception acc += arr[i]; }

but the IBM compiler will lift this out (under certain circumstances -- for instance, if 'acc' is not in scope of the catch) to:

if 0 outside of arr bounds || 999 outside of arr bounds, throw exception for (int i=0; i<1000; i++) { // look ma, no checks acc += arr[i]; }

does GHC do anything similar to this, or are we getting hit with array checks at each and every read/write (modulo, of course, full laziness)?

No, we don't do anything that clever. Some of the bulk array operations are written in such a way to eliminate obviously-true bound checks (eg. listArray doesn't do a bound check for every element), but that's about it. Cheers, Simon