in reply to Re^2: Memoize::Expire oddity
in thread Memoize::Expire oddity

$ perl -MMemoize -MMemoize::Expire -E " \
    sub f { int rand 100 }; \ 
    tie my %cache => 'Memoize::Expire', LIFETIME => 2; \
    memoize 'f', LIST_CACHE => 'MERGE', \
                 SCALAR_CACHE => [ HASH => \%cache ]; \
    for (1..10) { say f(); sleep 1 }"
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Re^4: Memoize::Expire oddity
by Yary (Pilgrim) on Aug 29, 2013 at 15:18 UTC
    Thanks for this example. With a little further modification it demonstrates that Memoize::Expire won't work for functions returning lists: 
    perl -MMemoize -MMemoize::Expire -E " \
    sub f { ('X',int rand 100) }; \ 
    tie my %cache => 'Memoize::Expire', NUM_USES => 3; \
    memoize 'f', LIST_CACHE => 'MERGE', \
                 SCALAR_CACHE => [ HASH => \%cache ]; \
    for (1..6) { say f() }"

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    You should be able to use any tied hash- not only Memoize::Expire but also any of the file-stores- if you first wrap it in something that will freeze/thaw the list beforehand. Example: 
    perl -MMemoize -MMemoize::Expire -MSerializeValues -E " \
     sub f { ('X',int rand 100) }; \
     tie my %subcache => 'Memoize::Expire', NUM_USES => 3; \
     tie my %cache => 'SerializeValues', \%subcache; \
     memoize 'f', LIST_CACHE => [ HASH => \%cache ], \
                  SCALAR_CACHE =>'MERGE' ; \
     for (1..6) { say f() }"

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    Below is my SerializeValues wrapper. Note that it seems to call the sub-cache's "FETCH" while it stores, effectively reducing NUM_USES by one. Also since Serialize requires a reference, it has to be in the LIST_CACHE slot, not SCALAR_CACHE. 
    #!/usr/bin/env perl

package SerializeValues;
require Tie::Hash;
 @ISA = qw(Tie::ExtraHash);

use strict;
use warnings;
use Storable qw[freeze thaw];


sub TIEHASH {
  my ($class, $child_hash) = @_;
  return bless [$child_hash], $class;
}

sub STORE {
  $_[0][0]{$_[1]}= freeze $_[2]
}

sub FETCH {
  thaw $_[0][0]{$_[1]}
}

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    I'm going to email the author (mjd) and suggest adding a flag to the HASH option to say that the values should be serialized first.

    PS. This module is over a decade old, and within a couple weeks two of us independently run into the same limitation- there's something going around...

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      Thanks for this example. With a little further modification it demonstrates that Memoize::Expire won't work for functions returning lists
      Right, which is why when I needed such abilities I wrote my own expiring backend to Memoize. If you want to do the same, start with the INTERFACE section of the Memoize::Expire POD.

      For the record, I also had to implement FIRSTKEY and NEXTKEY methods for things to work correctly.

      Cheers.
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        Ah, well... my little example above lets you use any existing tied hash with Memoize in list context, and I'm using it successfully with real code backed by Memoize::Expire in once case and and AnyDBM_File in another. Just a few lines, nothing reimplemented!
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