From: "Shot (Piotr Szotkowski)" <shot@...>
Date: 2009-09-18T04:56:17+09:00
Subject: How can I cache param-based Enumerator call results?

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I=E2=80=99m trying to cache a lazy-evaluated, param-based Enumerator calls.

Considering the following code:


def arith a, b
  Enumerator.new do |yielder|
    ['+', '*'].each do |oper|
      sleep 1 # simulates a param-based, long-running process
      yielder.yield [oper, eval("#{a} #{oper} #{b}")]
    end
  end
end

[[1,2], [3,4], [5,6], [1,2], [3,4], [5,6], [7,8]].each do |a, b|
  arith(a,b).each do |oper, result|=20
    puts "#{a} #{oper} #{b} =3D #{result}"
  end
end


should take about 14 seconds to run, with arith() lazily returning the
sum and product of its params. Still, if you look closely, arith(1,2),
arith(3,4) and arith(5,6) are called twice; if there was caching of
the yielded values there would be no need to recompute these sums
and products (and the whole process would take around eight seconds
instead).

I=E2=80=99m still rather new to Enumerators, but in my project their lazy
evaluation is an amazing benefit. What=E2=80=99s the best/simplest way to
introduce caching in the above scenario?

(Ideally, I=E2=80=99d love to have the caching done outside the Enumerator,
as in my project=E2=80=99s case the arith() method is provided by various
interchangeable modules=C2=B9, but if it=E2=80=99s only =E2=80=93 or much e=
asier =E2=80=93 doable
=66rom inside the Enumerator then so be it, I=E2=80=99ll add it in every
implementation=E2=80=A6)

=C2=B9 http://github.com/Chastell/art-decomp/blob/master/lib/art-decomp/dec=
omposer.rb
=E2=80=93 ignoring that #decompositions is itself an Enumerator, what I wan=
t is
for qv_gen to not recompute blankets if qv_gen.blankets(fsm, u, v, qu)
was already called previously with the same params

Thanks in advance for any insight in how to solve this problem!

=E2=80=94 Shot
--=20
IMO, the primary historical significance of Unix is that
it marks the time in computer history where CPUs became
so cheap that it was possible to build an operating
system without adult supervision.   [Russ Holsclaw, afc]

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