use Perl

• Odd - Pug's state worked correctly back in 2005 (Score:1)

  by Limbic Region (3985)

  See http://svn.pugscode.org/pugs/examples/cashiers.pl [pugscode.org]

  # Demo of the state() variable declaration.
  # This is also a neat way of doing OO without actually having OO available.
  #
  # Please remember to update t/examples/examples.t and rename
  # examples/output/cashiers if you rename/move this file.
   
  use v6-alpha;
   
  sub gen_cashier () {
      # This variable corresponds to a class variable.
      # It is shared across all "instances" of gen_cashier().
      state $cash_in_store = 0;
  
  • Re:Odd - Pug's state worked correctly back in 2005 (Score:2)

    by nicholas (3034) <nwc10+useperl@colon.colondot.net> on 2008.04.29 9:02 (#62465) Homepage Journal

    I don't think that it did. Certainly, that example is not a good enough test, because it makes all three calls to gen_cashier before it uses any of the closures, so it makes no difference how many times that assignment of 0 is performed. If I re-write it like this
    

    use v6-alpha;
    
    sub gen_cashier () {
        # This variable corresponds to a class variable.
        # It is shared across all "instances" of gen_cashier().
        state $cash_in_store = 0;
    
        # One could add my() variables here, which correspond to instance variables.
        # These would not be shared.
    
        # Finally, we return a hashref which maps method names to code.
        return {
            add => { $cash_in_store += $^amount },
            del => { $cash_in_store -= $^amount },
            bal => { $cash_in_store             },
        };
    }
    
    gen_cashier()<add>( 59 );
    gen_cashier()<del>( 17 );
    say gen_cashier()<bal>();  # This should say "42"

    
    then I get the wrong answer:
    

    $ ./pugs examples/cashiers.pl
    0

    If I remove the assignment of 0, and rely on undef becoming 0:
    

    sub gen_cashier () {
        # This variable corresponds to a class variable.
        # It is shared across all "instances" of gen_cashier().
        state $cash_in_store;
    
        # One could add my() variables here, which correspond to instance variables.
        # These would not be shared.
    
        # Finally, we return a hashref which maps method names to code.
        return {
            add => { $cash_in_store += $^amount },
            del => { $cash_in_store -= $^amount },
            bal => { $cash_in_store             },
        };
    }
    
    gen_cashier()<add>( 59 );
    gen_cashier()<del>( 17 );
    say gen_cashier()<bal>();  # This should say "42"

    
    then I get back to the right answer:
    

    ./pugs -w examples/cashiers.pl
    42

    state is doing something though here - a single variable is being shared between all 3 closures. To get the same effect with my I have to hoist my $cash_in_store outside of sub gen_cashier

    In conclusion, I don't think that state in Pugs has regressed - it never worked quite as well the example implied.

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