in reply to Re: How Does Interpolation Work?
in thread How Does Interpolation Work?

Oh, you asked for it!

Mark-Jason Dominus presented this at the Perl Conference. It's a generic way to evaluate a Perl expression inside a double-quoted string:

package Eval;

sub import
{
    my ($package, $name) = @_;
    $name = 'Eval' unless defined $name;
    my %magical_hash;
    tie %magical_hash =>Eval;
    my $caller = caller;
    
    *{caller . '::' . $name} = \%magical_hash;
}

sub TIEHASH
{
    my $self = \'fake';
    bless $self => Eval;
}

sub FETCH 
{
    my ($self, $value) = @_;
    $value;
}

[download]

Looks pretty freaky, eh? Okay, now try this:

use Eval => ':';

$salary = 43_000;
print "After your raise, you will make $:{$salary*1.06}.\n";

[download]

Yeek. Have fun!

Replies are listed 'Best First'.
RE: RE: Re: How Does Interpolation Work? (CMonster: method call eval)
by jeorgen (Pilgrim) on Jul 22, 2000 at 14:45 UTC
    Thanks for the example, CMonster, but I don't get it to work on my machine (ActiveState 5.005_03).

    I get:

    Warning: Use of "caller" without parens is ambiguous at C:\... line 11. syntax error at C:\... line 29, near "use Eval =>"

    So I change from:

    use Eval => :;

    to:

    use Eval ( ':');

    at line 29.

    Then I get:

    Modification of a read-only value attempted at Eval.pm line 17

    and that one I just don't know what to do about. I tried to change

    my $self = \'fake'

    to:

    my $self = {}

    at line 16, but that didn't help.

    /jeorgen

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      I had to make the following changes:
      • Add a $ to the typeglob line: *{$caller . '::' . $name} = \%magical_hash;
      • Do a two-step reference creation in TIEHASH(): 
        my $self = 'fake';
$self = \$self;

        [download]
      • Use a more standard use statement: use Eval (':');
      It won't run under strict, though, with the symbolic reference in the import() bit.
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