Luke Palmer wrote:
On Fri, Aug 28, 2009 at 6:04 AM, Bulat Ziganshin wrote:
it looks like cache of values computed since the last GC, because on GC all those intermediate results will be collected. i think it's not very useful outside of fib example that does exact that - reusing recently computed values
I wouldn't be so quick to call it useless. This caching idea, when combined with HNF instead of WHNF reduction, leads to a strategy which is capable of automatically specializing away layers of interpretation. That is to say, it turns all interpreters into compilers.
Now, there are some disadvantages too, some of which have to do with memory performance. But it's not clear that the disadvantages always outweigh the advantages; rather I suspect you get a strategy which implies a whole different set of trade-offs for engineering efficient programs.
Indeed. The fibs example highlights the biggest place where memoization wins: dynamic programming. DP can dramatically reduce the asymptotic complexity of all sorts of recursive algorithms. The idea of automatic pervasive memoization (done right) is nothing short of teaching the compiler to do DP internally. The analogy between laziness and memoization is so apt because laziness ala GHC is one restricted version of memoization. Both can improve asymptotic complexity over naive code, or can introduce undesired bloating. Both can improve clarity by saying what you mean, or can lead to inscrutable program performance depending on the whims of the Sufficiently Smart Compiler and the programmer's knowledge of its guts. Many of the laziness quirks about how to order traversals show up in spades with DP. (In fact, converting an algorithm into a DP version often amounts to 'just' reordering the traversals.) In Dyna we were working on automatic pervasive memoization from the perspective of Prolog and inductive databases. That approach works particularly well for a lot of natural language processing tasks, but it's biting off a lot (and inductive databases went the way of artificial intelligence because of that complexity). I'd be interested in seeing a language like Luke describes, which considers APM more from the direction of laziness and partial compilation. Putting the two together would make for a whole new era in programming (or at least an awesome language for academe). -- Live well, ~wren