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
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