On Fri, Aug 19, 2011 at 09:43:13AM -0700, Tom Murphy wrote:
On Fri, Aug 19, 2011 at 2:52 AM, Daniel Fischer < daniel.is.fischer@googlemail.com> wrote:
[...] In most cases, you'll get recomputation, but GHC does a bit of CSE, so in some cases it will compute only once and share the result (which may be a bad thing - that's a further reason for not doing too much CSE, sometimes sharing has catastrophic results).
This is a beginner's list, so I'll ask: what catastrophic result could it have, if it's computing pure functions?
It cannot change the meaning of a program. But it can drastically change the memory usage. For example, consider (length [1..1000000], sum [1..1000000]) vs. let x = [1..1000000] in (length x, sum x) The first expression needs only a constant amount of memory, since each list can be incrementally generated and garbage collected while accumulating the length and sum. In the second program, however, x cannot be garbage collected while length is being computed, since x is still being referred to by sum. So at the point just after evaluating length and before starting in on evaluation of sum, the entire million-element list is in memory. -Brent